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`.

editor/export/export_template_manager.cpp

@@ -33,6 +33,7 @@
 #include "core/io/dir_access.h"
 #include "core/io/json.h"
 #include "core/io/zip_io.h"
+#include "core/templates/rb_set.h"
 #include "core/version.h"
 #include "editor/editor_node.h"
 #include "editor/editor_string_names.h"