Create a custom plugin to centralize UI setup

The UI elements need some things set up for them to work properly.
Namely, the `UiTheme` resource and the Observers which handle button
presses.

src/main.rs

@@ -20,8 +20,7 @@ fn main() {
     }))
     .add_plugins(EguiPlugin::default())
     .add_plugins(WorldInspectorPlugin::new())
-    .init_resource::<resources::UiTheme>()
+    .add_plugins(widgets::GameUiPlugin)
-    .register_type::<resources::UiTheme>()
     .add_systems(
       Startup,
       (
@@ -31,10 +30,6 @@ fn main() {
         CuttingMachine::spawn_ui,
       ),
     )
-    .add_observer(widgets::CloseButton::hover_start)
-    .add_observer(widgets::CloseButton::hover_stop)
-    .add_observer(widgets::CloseButton::press_start)
-    .add_observer(widgets::CloseButton::press_stop)
     .run();
 }
 

src/widgets/mod.rs

@@ -6,6 +6,24 @@ use bevy::{color::palettes::css::*, prelude::*, ui::Val::*};
 
 use crate::resources::UiTheme;
 
+/// Plugin to set up systems & resources for the custom UI elements
+///
+/// The UiTheme resource is initialized and the button press Observers
+/// are registered here.
+pub struct GameUiPlugin;
+
+impl Plugin for GameUiPlugin {
+  fn build(&self, app: &mut App) {
+    app
+      .init_resource::<UiTheme>()
+      .register_type::<UiTheme>()
+      .add_observer(CloseButton::hover_start)
+      .add_observer(CloseButton::hover_stop)
+      .add_observer(CloseButton::press_start)
+      .add_observer(CloseButton::press_stop);
+  }
+}
+
 /// 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