Add interactivity to Checkbox component (#3240)

This PR adds interactivity to the `Checkbox` component.

They can now be checked and unchecked by clicking them.

Release Notes:

- N/A

crates/ui2/src/components/button.rs

@@ -61,7 +61,7 @@ impl ButtonVariant {
     }
 }
 
-pub type ClickHandler<S> = Arc<dyn Fn(&mut S, &mut ViewContext<S>) + Send + Sync>;
+pub type ClickHandler<V> = Arc<dyn Fn(&mut V, &mut ViewContext<V>) + Send + Sync>;
 
 struct ButtonHandlers<V: 'static> {
     click: Option<ClickHandler<V>>,