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.

core/engine.h

@@ -31,10 +31,10 @@
 #ifndef ENGINE_H
 #define ENGINE_H
 
-#include "list.h"
-#include "os/main_loop.h"
-#include "ustring.h"
-#include "vector.h"
+#include "core/list.h"
+#include "core/os/main_loop.h"
+#include "core/ustring.h"
+#include "core/vector.h"
 
 class Engine {