Collect GUI plugins at the top of the module

I want the different "scenes" to be their own plugins for ease of setup
and reading.

The main menu plugin has been renamed to have "Plugin" first. This is so
the lexical sort in the docs places all the plugins next to each other.

The "get-ready" plugin has been given an empty struct and an
`impl Plugin` to match the main menu plugin. I've started the game over
scene, but left it unimplemented.

src/lib.rs

@@ -34,8 +34,8 @@ pub struct AsteroidPlugin;
 impl Plugin for AsteroidPlugin {
     fn build(&self, app: &mut App) {
         app.add_plugins((
-            widgets::GameMenuPlugin,
-            widgets::preparation_widget_plugin,
+            widgets::PluginGameMenu,
+            widgets::PluginGetReady,
             RapierPhysicsPlugin::<NoUserData>::pixels_per_meter(10.0),
             RapierDebugRenderPlugin::default(),
         ))