clean up a few xu_* functions to just accept what they need (Window).

ok oga@

group.c

@@ -426,7 +426,7 @@ group_autogroup(struct client_ctx *cc)
 
 	if (cc->app_class == NULL || cc->app_name == NULL)
 		return;
-	if (xu_getprop(cc, _NET_WM_DESKTOP, XA_CARDINAL,
+	if (xu_getprop(cc->win, _NET_WM_DESKTOP, XA_CARDINAL,
 	    1, (unsigned char **)&grpno) > 0) {
 		if (*grpno == 0xffffffff)
 			no = 0;