finally implement keyboard binding for group toggling

idea for the
    "slightly-less-abhorrent-hack-but-a-hack-nonetheless-TM" from oga@

grab and ungrab the keyboard to get around some silly X apps that like
stealing events

ok oga@

client.c

@@ -438,6 +438,7 @@ client_unhide(struct client_ctx *cc)
 	XMapWindow(X_Dpy, cc->win);
 	XMapRaised(X_Dpy, cc->pwin);
 
+	cc->highlight = 0;
 	cc->flags &= ~CLIENT_HIDDEN;
 	xu_setstate(cc, NormalState);
 }