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

We want the ability to create a detached menu.
diff --git a/clients/window.h b/clients/window.h
index b7b3f6a..1d0c4af 100644
--- a/clients/window.h
+++ b/clients/window.h
@@ -284,8 +284,7 @@
 int
 window_has_focus(struct window *window);
 
-typedef void (*menu_func_t)(struct window *window,
-			    struct input *input, int index, void *data);
+typedef void (*menu_func_t)(void *data, struct input *input, int index);
 
 void
 window_show_menu(struct display *display,