Implement menu despawner

client/src/ui.rs

@@ -46,8 +46,16 @@ pub fn spawn_main_menu(mut commands: Commands) {
         });
 }
 
-pub fn despawn_main_menu(mut commands: Commands) {
-    warn!("->> ui.rs: despawn_main_menu() is not implemented.");
+/// Despawns the main menu (which is assumed to be the top-most node)
+///
+/// TODO: Add a marker component, but only in debug builds. A unit test can
+/// assert this condition so I don't actually have to check it at runtime
+/// on release builds.
+pub fn despawn_main_menu(
+    mut commands: Commands,
+    top_node: Single<Entity, (With<Node>, Without<ChildOf>)>,
+) {
+    commands.entity(top_node.into_inner()).despawn();
 }
 
 /// The basic bundle for generic buttons.