Implement _NET_WM_STATE_STICKY, bound to CM-s by default; allows any

client to 'stick' to all desktops (ewmh speak) or groups - this
currently has the same affect as setting a client's group to 'nogroup',
with the exception that the client can also be in a group, so when
un-sticking, the client will go back to its original group/desktop.

group.c

@@ -207,6 +207,8 @@ group_hidden_state(struct group_ctx *gc)
 	int			 hidden = 0, same = 0;
 
 	TAILQ_FOREACH(cc, &gc->clients, group_entry) {
+		if (cc->flags & CLIENT_STICKY)
+			continue;
 		if (hidden == ((cc->flags & CLIENT_HIDDEN) ? 1 : 0))
 			same++;
 	}