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/settings_config_dialog.cpp

@@ -30,11 +30,11 @@
 
 #include "settings_config_dialog.h"
 
+#include "core/os/keyboard.h"
+#include "core/project_settings.h"
 #include "editor_file_system.h"
 #include "editor_node.h"
 #include "editor_settings.h"
-#include "os/keyboard.h"
-#include "project_settings.h"
 #include "scene/gui/margin_container.h"
 #include "script_editor_debugger.h"