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.

scene/resources/shape_2d.h

@@ -31,7 +31,7 @@
 #ifndef SHAPE_2D_H
 #define SHAPE_2D_H
 
-#include "resource.h"
+#include "core/resource.h"
 
 class Shape2D : public Resource {
 	GDCLASS(Shape2D, Resource);