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@

kbfunc.c

@@ -429,6 +429,16 @@ kbfunc_client_nogroup(struct client_ctx *cc, void *arg)
 	group_alltoggle();
 }
 
+void
+kbfunc_client_grouptoggle(struct client_ctx *cc, void *arg)
+{
+	/* XXX for stupid X apps like xpdf and gvim */
+	XGrabKeyboard(X_Dpy, cc->pwin, True,
+	    GrabModeAsync, GrabModeAsync, CurrentTime);
+
+	group_sticky_toggle_enter(cc);
+}
+
 void
 kbfunc_client_maximize(struct client_ctx *cc, void *arg)
 {