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.

main/tests/test_gui.cpp

@@ -32,9 +32,9 @@
 
 #include "test_gui.h"
 
-#include "io/image_loader.h"
-#include "os/os.h"
-#include "print_string.h"
+#include "core/io/image_loader.h"
+#include "core/os/os.h"
+#include "core/print_string.h"
 #include "scene/2d/sprite.h"
 #include "scene/gui/button.h"
 #include "scene/gui/control.h"