Formspec: change tabs with ctrl+tab

[siliconsniffer]

This PR adds the ability to switch to the next/previous tab using ctrl(+shift) +tab.

Note: I have used AI during development.

• Goal of the PR
  Make it easier/quicker to go to the next/previous tab using keyboard controls.

• How does the PR work?
  It modifies the GUIFormSpecMenu's OnEvent method to cycle through tabs using the above mentioned keyboard shortcut.

  There are also a few event handling refinements:

  • Updated GUIFormSpecMenu's preprocessEvent method to prevent Ctrl+Tab and Return keys from being incorrectly consumed by child elements like checkboxes, tables, and list boxes.
  • Modified GUITable to pass key events with the Control modifier to its parent, ensuring consistent event propagation.
  • Adjusted GUITable's OnEvent method to no longer consume the Space key, allowing it to pass to the parent element instead.

• Does it resolve any reported issue?
  Not any that I am aware of.

• Does this relate to a goal in the roadmap?
  Hopefully 2.3.

• If not a bug fix, why is this PR needed? What usecases does it solve?
  It makes it easier and quicker to go to the next or previous tab.

To do

This PR is ready for Review.

• Add demo video?
• Discuss changes regarding event handling but unrelated to the tab cycling

Preview

2025-05-26.20-05-54.mp4

How to test

Open Luanti → try switching forth and back using the keyboard shortcut.

[SmallJoker]

Quick review. I like this feature and also wonder why no-one else has yet come up with this...
Thank you.

[siliconsniffer]

Thanks for the review and the kind words. I was wondering the same!
It's really a small but nice QoL improvement and somewhat an "industry standard" already.

Edit: The only issue I found can be seen e.g. in devtest in the "Tabs" tab, where instead of the test_formspec tabs it will focus the tabs inside of that page. Similar behavior happening in the "Real Coord" tab.

[SmallJoker]

The tab control is not always selected correctly, but generally there's at most one, which makes it tolerable.
I tried to fix that by checking the parent of the focused element - but tab controls are apparently only used for visuals and not to actually contain elements (as it would be intended by the Irrlicht API). I suppose the current solution must suffice.

[siliconsniffer]

I tried to fix that by checking the parent of the focused element - but tab controls are apparently only used for visuals and not to actually contain elements (as it would be intended by the Irrlicht API). I suppose the current solution must suffice.

Isn't that worth opening an issue for (maybe even just to document it)?

[SmallJoker]

Isn't that worth opening an issue for [...]?

It's an implementation detail. Formspecs could be made more responsive but I wonder whether that would justify additional traffic (to send all tab contents in the same formspec),.
One could perhaps open an issue about that, but I honestly don't care, thus no action from my side.