Fix: improve appearance of toolbars and main menu images/text

[PeterN]

Motivation / Problem

The images used on the main menu are highly dependent on the baseset used, and currently rely on all images being the same size.

This unfortunately doesn't hold for all basesets, most notably the original TTD baseset, and this results in the text of the main menu not being aligned neatly.

Part of the issue is that the method to set button size relative to toolbar button sizes uses a hardcoded pixel size value, which may not actually match the loaded baseset.

This is noticeable when using a graphics set such as "aBase" or "biggui", which use larger than intended toolbar icons, but only for some toolbar icons, resulting in a weird mix of sizes.

Description

These issues are resolved by making toolbar-button-sizes relative to the largest normal toolbar icon, instead of the hardcoded 20 pixels, and then forcing the image part of image-text buttons to be square, based on toolbar button sizing.

This is not 100% ideal for image-text buttons, but there are currently no other users of this widget type so it's mostly fine.

It's still not really recommended to use larger icons than designed, as it will always cause inconsistency with NewGRF addons which are generally designed for the default button size.

Limitations

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR touches english.txt or translations? Check the guidelines
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, game_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker