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.cpp

@@ -29,9 +29,10 @@
 /*************************************************************************/
 
 #include "physics_2d_server.h"
+
 #include "core/method_bind_ext.gen.inc"
+#include "core/print_string.h"
 #include "core/project_settings.h"
-#include "print_string.h"
 
 Physics2DServer *Physics2DServer::singleton = NULL;