Implement the ability to disable classes

* This PR adds the ability to disable classes when building.
* For now it's only possible to do this via command like:
  `scons disable_classes=RayCast2D,Area3D`
* Eventually, a proper UI will be implemented to create a build config file to do this at large scale, as well as detect what is used in the project.

modules/mobile_vr/register_types.cpp

@@ -33,7 +33,7 @@
 #include "mobile_vr_interface.h"
 
 void register_mobile_vr_types() {
-	ClassDB::register_class<MobileVRInterface>();
+	GDREGISTER_CLASS(MobileVRInterface);
 
 	if (XRServer::get_singleton()) {
 		Ref<MobileVRInterface> mobile_vr;