Make core/ includes absolute, remove subfolders from include path

This allows more consistency in the manner we include core headers,
where previously there would be a mix of absolute, relative and
include path-dependent includes.

servers/physics_2d_server.h

@@ -31,9 +31,9 @@
 #ifndef PHYSICS_2D_SERVER_H
 #define PHYSICS_2D_SERVER_H
 
-#include "object.h"
-#include "reference.h"
-#include "resource.h"
+#include "core/object.h"
+#include "core/reference.h"
+#include "core/resource.h"
 
 class Physics2DDirectSpaceState;