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`.
- The panel’s layout and position are now saved and restored across projects and editor restarts.
- Toggle buttons for Ctrl, Shift, and Alt keys have been added to support actions that require holding down these keys, such as Ctrl for multi-selection.
- Additional action buttons have been introduced: Cut, Copy, and Paste.
- Some UI polish.
