In most cases, we may only need to modify some properties of the default
resource instead of all. But click the plus icon to override item, it
will use an empty object to override. Allow `EditorResourcePicker` to be
editable, you can use the **Make Unique** option to create a copy and
then edit it.
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.