This allows removing it from `class_db.h`, significantly reducing the amount of files
that include it transitively.
Also includes some include cleanup in `control.h` and `rich_text_label.h` done while
ensure they don't depend on `callable_mp`.
- Correctly display atlas textures when used by other objects.
- Make region handles easier to hit in ninepatchable objects.
- Correctly initialize and restore various visual properties.
- Improve code quality.
Rémi Verschelde
Thaddeus Crews
kobewi
A Thousand Ships
kleonc
Yuri Sizov
Hendrik Brucker