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

We want the ability to create a detached menu.
diff --git a/clients/resizor.c b/clients/resizor.c
index 029042f..19c6eeb 100644
--- a/clients/resizor.c
+++ b/clients/resizor.c
@@ -200,8 +200,7 @@
 }
 
 static void
-menu_func(struct window *window,
-	  struct input *input, int index, void *user_data)
+menu_func(void *data, struct input *input, int index)
 {
 	fprintf(stderr, "picked entry %d\n", index);
 }