This will allow decoupling `display_server.h` from a number of headers in the
codebase which only require those enums and not all the DisplayServer API.
- 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`.
Currently Unittests simplify mouse-events by just pushing them to Viewports.
For dealing with mouse-screen-coordinates (caused by the introduction of
multiple native Windows) it becomes necessary to extend the
DisplayServer functionality for unittests.
This PR introduces DisplayServerMock based on DisplayServerHeadless,
which additionally supports basic Mouse-Input handling.
