GUI elements ui_action usage, improvements

[Faless]

• Popupmenu now uses UI actions instead of key presses
• Tree now uses UI actions instead of key presses
• Line edit up/down focus pass through
  When line edit receive a up/down and the cursor is at beginning/end it will not set the input as handled
• Properly sets input as handled when closing modal
• Scrollbar now uses UI actions instead of key presses
• Add two new default actions ui_end, ui_home, used by Slider and Scrollbar (I can remove this if found useless)
• Add item_focused signal to OptionButton (and id_focused to PopupMenu)
  This is useful if for example you want to add UI sounds to your game.

Closes #16946
Closes #17159

[vnen]

This fixes #16946.

[Faless]

thanks, added it to the PR description so it should get closed automatically when merged

[bfloch]

Awesome. Thanks.

[bfloch]

I've tested this and it seems to work.

I still have an issue where a popup panel does not react to any focus events on a keyboard. I try to find out if this is related or a separate issue.

[Faless]

As no one seems to find the courage to merge this even though it's been tested and approved by a few, I'll abuse my newly acquired power and do it myself 😁

[Faless]

I'm sure there will be plenty of time to take the blame before 3.1 is out if something breaks 😼

[legione]

Is there any chance to see these fixes in 3.0.3?
I don't think these changes would break the compatibility and half of them are bugfixes anyway.

[legione]

May I also point to #18715 that I think is related?

[follower]

Hi @Faless, would you be open to revisiting this code to see if you can fix the issues with keyboard navigation that seemed to occur as a result of the "Tree now uses UI actions instead of key presses" changes?

Assuming my assessment of the situation is correct, the issues have had a significant negative impact on the accessibility & usability of the Tree control both within the editor & third-party games/apps.

There's some further details in #41014 & #36291.

Thanks!