Trait SpawnUi for things that can spawn a UI

I don't want to copy-paste a bunch of code, so I'm going to get the
compiler to do it for me.

src/widgets/mod.rs

@@ -28,6 +28,14 @@ impl Plugin for GameUiPlugin {
   }
 }
 
+/// Anything that can spawn a themed UI should impl this trait.
+///
+/// This exists mainly so that I can write generic functions and have the *compiler*
+/// expand the code for me, instead of doing it by hand.
+pub trait SpawnUi {
+  fn spawn_ui(commands: Commands, theme: Res<UiTheme>);
+}
+
 /// The base panel for the machines that manipulate the room cards.
 ///
 /// This function is not a valid Bevy System because of how the Commands struct