Fix always_on_top for sub-dialogs

[Variable-ind]

Fix Create Folder sub-dialog of Export > Browse being always behind (thus making it unusable).

This shall fix the problem on other locations as well. As long as they are included in the FileDialogs tag (in Main.tscn)

[Variable-ind]

I noticed there are some other cases where the popups of option buttons get hidden behind the window if the window has always_on_top enabled. (like the LayoutFrom option button in ManageLayouts.tscn)

so i'm converting this to a draft and will modify this later to include those fixes as well

[Variable-ind]

Okay, so i managed to fully fix it
(Some comments about the PR):

• FixInternalDialogs is a dedicated group which only contains items which need correction (once they are corrected they are removed from the group).
• FixInternalDialogs is a Global group instead of a Scene group.
  • This is to avoid the hassle of creating the same group again and again for each scene (Plus they can be change to scene groups later anyway, if the need arises).
  • This also make it easier to undo this PR if it gets fixed in Godot (i checked in Godot 4.4 but it wasn't fixed yet)
• The code for correction is moved to the Dialog class because:
  • (Short version): It's a more general solution. Items with internal popups/windows can be placed in FixInternalDialogs group and the system automatically fixes them (even if their parent window is a Dialog class). This isn't (easily) possible if the code is moved anywhere else.
  • (Long version) Even if currently only Export and Save dialogs have this bug (i haven't seen it anyplace else, besides the now removed ManageLayouts.tscn), we may in the future encounter this in one of the Dialogs (like we did in the LayoutFrom OptionButton of ManageLayouts.tscn), And since Dialogs are not "immediately" part of the scene, we have to fix it's internal dialogs WHILE it's being added to tree.

Related stuff:
godotengine/godot#95096
godotengine/godot#92848

[OverloadedOrama]

Thanks!