Add on_release message to Slider (#378)

* Add on_finish callback to Slider

* Fix formatting

* Rename Slider's on_finish to on_release, make the message simply an event without data

* Satisfy Clone impl requirement on Message in integration test

* Only call on_release after dragging a slider

examples/integration/src/controls.rs

@@ -9,7 +9,7 @@ pub struct Controls {
     sliders: [slider::State; 3],
 }
 
-#[derive(Debug)]
+#[derive(Debug, Clone)]
 pub enum Message {
     BackgroundColorChanged(Color),
 }