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`.
Replace text for add/close buttons with icons, remove theme preview custom minimum width, fix "Manage Items" window size in single-window mode and editor window at minimum size. Also theme preview tab looks like scenes tab.
This allows to reduce minimum size of `ThemeEditor`, improving (partially fixing) the use of non-standard editor layouts (for example, 2 columns for left dock and 1 column for right dock) without panels pushed out of screen.
StarryWorm
Rémi Verschelde
Thaddeus Crews
Michael Alexsander
kobewi
arkology
A Thousand Ships