Both the native and the web slider now have a member `step` to control
the least possible change of the slider's value. It defaults to 1.0
for all sliders and can be adjusted with the step method.
* 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
