clients: Remove the window / user parameters from the menu function

We want the ability to create a detached menu.
diff --git a/clients/stacking.c b/clients/stacking.c
index f51a476..abeded8 100644
--- a/clients/stacking.c
+++ b/clients/stacking.c
@@ -82,7 +82,7 @@
 }
 
 static void
-show_popup_cb(struct window *window, struct input *input, int index, void *data)
+show_popup_cb(void *data, struct input *input, int index)
 {
 	/* Ignore the selected menu item. */
 }