Clean up include macros mess

The order and way include macros were structured was chaotic, with no
real common thread between files. Try to tidy up the mess with some
common scheme, to make things look less messy.

src/core/keybindings-private.h

@@ -28,9 +28,10 @@
 #define META_KEYBINDINGS_PRIVATE_H
 
 #include <gio/gio.h>
-#include <meta/keybindings.h>
 #include <xkbcommon/xkbcommon.h>
-#include "meta-accel-parse.h"
+
+#include "core/meta-accel-parse.h"
+#include "meta/keybindings.h"
 
 typedef struct _MetaKeyHandler MetaKeyHandler;
 struct _MetaKeyHandler