Minimize include dependencies of display_server.h

- 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`.

tests/test_main.cpp

@@ -31,6 +31,7 @@
 #include "test_main.h"
 
 #include "core/error/error_macros.h"
+#include "core/input/input.h"
 #include "core/io/dir_access.h"
 #include "modules/modules_enabled.gen.h"