_MUTTER_HINTS

A per-window _MUTTER_HINTS property allowing plugins to use custom hints. The
property holds a colon separated list of key=value pairs; plugin-specific keys
must be suitably namespaced, while 'mutter-' prefix is reserved for internal
Mutter use only.

This commit adds MetaWindow::mutter-hints property, and
meta_window_get_mutter_hints() accessor, as well as the internal machinery for
reading and updating of the hints.

https://bugzilla.gnome.org/show_bug.cgi?id=613123

src/core/window-private.h

@@ -96,6 +96,7 @@ struct _MetaWindow
   char *sm_client_id;
   char *wm_client_machine;
   char *startup_id;
+  char *mutter_hints;
 
   int net_wm_pid;