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.

editor/rename_dialog.h

@@ -37,8 +37,8 @@
 #include "scene/gui/option_button.h"
 #include "scene/gui/spin_box.h"
 
+#include "core/undo_redo.h"
 #include "editor/scene_tree_editor.h"
-#include "undo_redo.h"
 
 /**
 @author Blazej Floch