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/gui/file_dialog.cpp

@@ -29,8 +29,8 @@
 /*************************************************************************/
 
 #include "file_dialog.h"
-#include "os/keyboard.h"
-#include "print_string.h"
+#include "core/os/keyboard.h"
+#include "core/print_string.h"
 #include "scene/gui/label.h"
 
 FileDialog::GetIconFunc FileDialog::get_icon_func = NULL;