Introduce a check to see if a group holds only 'sticky' clients and use

this check to decide if a group is virtually empty.  Rationale: if a
group contains *only* 'sticky' clients, it should be skipped while
cycling through groups.  Apply similar logic to the group menu.

Based on an idea from phessler@, who also tested another version.

mousefunc.c

@@ -188,7 +188,7 @@ mousefunc_menu_group(struct client_ctx *cc, union arg *arg)
 
 	TAILQ_INIT(&menuq);
 	TAILQ_FOREACH(gc, &sc->groupq, entry) {
-		if (TAILQ_EMPTY(&gc->clientq))
+		if (group_holds_only_sticky(gc))
 			continue;
 		menuq_add(&menuq, gc,
 		    group_hidden_state(gc) ? "%d: [%s]" : "%d: %s",