Move DisplayServer enums and typedefs to DisplayServerEnums

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.

servers/rendering/rendering_server_default.cpp

@@ -412,7 +412,7 @@ void RenderingServerDefault::_thread_exit() {
 }
 
 void RenderingServerDefault::_thread_loop() {
-	DisplayServer::get_singleton()->gl_window_make_current(DisplayServer::MAIN_WINDOW_ID); // Move GL to this thread.
+	DisplayServer::get_singleton()->gl_window_make_current(DisplayServerEnums::MAIN_WINDOW_ID); // Move GL to this thread.
 
 	while (!exit) {
 		WorkerThreadPool::get_singleton()->yield();