Make CloseButton observer functions private

Now that there's a UI plugin, these systems don't need to be public. So
they won't be.

src/widgets/mod.rs

@@ -214,7 +214,7 @@ impl CloseButton {
     )
   }
 
-  pub fn hover_start(
+  fn hover_start(
     event: Trigger<Pointer<Over>>,
     // Get button background and border colors so we can change them.
     // Filter for *changed* interactions, and only entities with a [`Button`]
@@ -228,7 +228,7 @@ impl CloseButton {
     }
   }
 
-  pub fn hover_stop(
+  fn hover_stop(
     event: Trigger<Pointer<Out>>,
     mut button_colors: Query<(&mut BackgroundColor, &mut BorderColor), With<CloseButton>>,
     ui_theme: Res<UiTheme>,
@@ -239,7 +239,7 @@ impl CloseButton {
     }
   }
 
-  pub fn press_start(
+  fn press_start(
     event: Trigger<Pointer<Pressed>>,
     mut button_colors: Query<(&mut BackgroundColor, &mut BorderColor), With<CloseButton>>,
     ui_theme: Res<UiTheme>,
@@ -250,7 +250,7 @@ impl CloseButton {
     }
   }
 
-  pub fn press_stop(
+  fn press_stop(
     event: Trigger<Pointer<Released>>,
     mut commands: Commands,
     mut button_colors: Query<(&mut BackgroundColor, &mut BorderColor, &CloseButton)>,