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/bell.h

@@ -17,8 +17,8 @@
  * along with this program; if not, see <http://www.gnu.org/licenses/>.
  */
 
-#include "display-private.h"
-#include "frame.h"
+#include "core/display-private.h"
+#include "core/frame.h"
 
 struct _MetaBell
 {