- Removes `native_menu.h` dependency from `display_server.h`.
It's now forward-declared in all DisplayServer implementations and should
be included in the .cpp's.
- Removes some unused `rb_set.h` and `rb_map.h` dependencies, which leads to
having to include them explicitly in half the scene and editor codebase...
which shows how much we depend on `display_server.h`.
- Forward-declare `input_event.h`, so now we need only `keyboard.h`.
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.