[3.x] Make popup menus focus items automatically when not using the mouse

[YeldhamDev]

Port of #56992 and #42843.

[akien-mga]

Thanks!

[RodZill4]

@YeldhamDev Hi! I have a problem with that change (but not fully investigated yet, so it's probably my fault). My project (Material Maker) has dynamic menus (built upon "about to show" events) and some are huge. When a dynamic submenu that is taller than the viewport is shown, it blinks and becomes unusable.
It seems moving the pm->popup() statement line 182 to before the if statement (where it was before your change) solves the problem. But for sure it's not the solution.
I tried to modify the code to display autohide areas, they are wrong (don't take into account that the submenu, being tall, is not placed next to the submenu entry). So the submenu item is in autohide zone, and that's why the menu blinks.
Weird thing is I tried to build a minimal example (dynamic tall menus) and it works fine. So I'm probably doing something wrong in Material Maker. Any clue? Something must be done in my gdscript code after filling the popup menu, maybe?
BTW, tall menus don't seem to scroll when navigating with keys.
(that comment is a mess, sorry)