Campaigns menu overhaul

[irydacea]

This overhauls the game's Campaigns menu following some of the directions previously suggested in issue #4543.

Requesting reviews from @Vultraz for the UI layout and code changes, @gfgtdf for the engine changes, and @Pentarctagon and @nemaara for the core changes to the single-player campaign UI flow.

User interface changes

• The main area of the dialog is rearranged into four distinct sections:
  • The caption row, with the dialog caption and search box
  • The main panel, with the list of campaigns on the left, the campaign description and image on the right, and an image background.
  • The settings row, with the Modifications, Combat (RNG) and Difficulty menus arranged in that order.
  • The control row, with the Play and Cancel buttons.
• The toggle buttons serving as the headers for the list of campaigns have their text centered in order to make the list header seem more tidy in general.
• The Difficulty menu is integrated into the dialog in the form of a dropdown menu button. The old standalone menu remains available for use by Load Game and others that may need it.
• The Save random seed option is replaced with a new Combat dropdown menu button allowing the player to choose between Default RNG, Predictable RNG (identical to Save random seed), and Biased RNG (identical to the Advanced Preferences experimental PRNG option, removed by this PR).

WML changes

• A new attribute, background=<image path> is added to top-level [campaign] to allow specifying a custom background image for the main panel. In the absence of this, a default image is used (currently story/landscape-battlefield_nohumans.jpg).
• The random_mode= attribute in game classification has a new possible value, biased, which selects the predictable RNG as with the Save random seed option before but also enables the RNG bias logic in the attack engine (previously the experimental PRNG option). The logic in the attack constructor may now be slightly redundant since it shouldn't be possible for random_mode to be biased and randomness::generator->is_networked() to return true (@gfgtdf may want to confirm this).
• toggle_button widgets gain new definitions: listbox_header_centered (unused), listbox_header_bg (unused), listbox_header_centered_bg (used by this PR for the campaign list header row).
• panel widgets gain new definitions: panel_listbox_bg (used by this PR for the campaign list header row), campaign_menu_bg (used by this PR for the main campaign menu panel).

Known issues

• The Difficulty menu button has to compose labels from the label proper specified in [difficulty] WML as well as the description in order for the labels to make sense (e.g. for HttT, the difficulty labels would be the rather meaningless "Fighter", "Commander", "Lord" otherwise). The problem arises from the possibility of [difficulty] descriptions using Pango markup that will look plain wrong on the menu button proper's label. We don't seem to have a function to unescape markup entities and strip Pango markup at the moment, so this is left unsolved in the PR for the time being.
• The Combat menu button has overlong descriptions resulting in a messy dropdown display as seen in the third screenshot above. I'm not entirely sure what to do about this without losing the intended conveyance of information — tooltips perhaps?
• The dialog has layout issues on low resolution (< 1024 px horizontal) modes and other situations where overlong text crops up.on the game settings row at the bottom. I want to fix this, ideally before merging.
• There is no way to set modification options from the dialog itself. If this is wanted it could be worked on at a later date, preferably with the help of someone who knows better how modifications work (such as @gfgtdf).
• The idea suggested in issue Campaign Menu GUI design  #4543 of adding a separate tab for tutorials is not implemented yet. There does not seem to be a use case for it yet either since the tutorial only has two scenarios at the moment. I also largely ignore if it's possible to instruct the game to skip to a scenario other than first_scenario. I would greatly appreciate some input on this particular point for this PR and others.
• Mainline campaigns have no custom backgrounds defined yet. Below is attached a patch I used to test the feature only, with no real thematic coherence intended ­— save perhaps for HttT, where my best option was the antiquated 1.0 story art used for the Battle for Wesnoth scenario.
• Campaign images (often portraits) are currently allowed to jump around even when the text doesn't take enough space for them to need to do so. Perhaps anchor them to the bottom of the right pane? (Note that @Vultraz seems to be of the opinion that we should do away with the images themselves. I do not agree with this.)
• Campaign descriptions may suffer from text contrast issues when a sufficiently bright background is selected. Currently this is partly alleviated by using a dark translucent overlay on the background (see the campaign_menu_bg definition for panel). I don't feel we can do anything about this short of adding shadows or (preferably) outlines to the text.
• Long campaign descriptions predictably cause a vertical scrollbar to appear. multi_page remembers the scrollbar position for each page and I can't quite decide if this is desirable behaviour or not.

Reference/test material

Original Experimental PRNG option patch

See commit 3f06813 and the forum thread, both by Dave.

Background image test patch

See https://gist.github.com/shikadiqueen/e66d433d9ee0d6dd4915f986d6658c38.

[nemaara]

Looks pretty nice.

[Vultraz]

One think that immediately stands out to me is that Modifications section. It should be aligned with the left window edge.

[soliton-]

Perhaps the combat descriptions can be moved to a linked help page instead.

[irydacea]

Perhaps the combat descriptions can be moved to a linked help page instead.

I thought about this, but that would mean adding a new help page and hoping for people to click on a somewhat unrelated element in the dialog (a new Help button) to figure out what the option does.

On the other hand, it turns out that tooltips do work with dropdown menu items as one would expect them to do without any issues:

[Pentarctagon]

@nemaara Once this is merged, can you take a look as far as adding backgrounds for the mainline campaigns?

[irydacea]

After consulting with @Pentarctagon and @Vultraz, we've decided to merge as-is. The PR is still here for @gfgtdf's reference if he becomes able to review the engine changes at a later time.

[AI0867]

I dislike the description of the Biased RNG. It seems to imply that the regular RNG doesn't obey the true probabilities, even though it doesn't actually say that.

I'm not sure how to word it better though.

[CelticMinstrel]

Now that this has been merged, I have two comments.

• First, I agree with @AI0867 on the description of the "Biased RNG". I'm not 100% how it works, but I did glance at the original commit and it looks similar to DotA's system where the chance to hit starts below the target chance but increases with every miss, resetting once there's a hit. I don't think "biased" is at all an accurate description of this - biased gives the impression that it's altering the probabilities, but if it is similar to what DotA does, then it's not altering the probabilities themselves, only the distribution of successes and failures. Perhaps "Smoothed RNG" or something similar? I'm not happy with that either but it seems less misleading than "Biased RNG".
• It seems like there's no longer any way to use the experimental PRNG without enabling Save Random Seed, is that correct? I'd prefer them to be two separate options. They have nothing to do with each other, after all.