Slider doesn't properly bubble the OnKeyDown event

Once a slider receives keyboard focus, the keys will not navigate away regardless of the navigation settings. This is because the slider's OnKeyDown function does not pass along the call when the event is unhandled.

1. Create a widget with a slider and some other elements
2. Set focus to one of your other elements
3. Navigate over to the slider
4. You will not be able to navigate away from the slider