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/progress_dialog.cpp

@@ -30,10 +30,10 @@
 
 #include "progress_dialog.h"
 
+#include "core/message_queue.h"
+#include "core/os/os.h"
 #include "editor_scale.h"
 #include "main/main.h"
-#include "message_queue.h"
-#include "os/os.h"
 
 void BackgroundProgress::_add_task(const String &p_task, const String &p_label, int p_steps) {