Fix timeline touch

[hpinkos]

Replaces #4286

_handleTouchMove was an old function that was removed (commit 1b1dea5).

Replacing with _onTouchMove allows the touch to work on the timeline.

To experience this bug:

1. Go to http://cesiumjs.org/Cesium/Apps/Sandcastle/index.html?src=Hello%20World.html&label=Showcases
2. Using touch (or an emulation in Chrome), touch the timeline bar (make sure it's touch, not a mouse).
3. Uncaught TypeError: timeline._handleTouchMove is not a function.

Here's how you can emulate touch on a desktop in Chrome

[pjcozzi]

@hpinkos can you please update CHANGES.md?

@lilleyse can you test this on the iPad when you have a minute?

[hpinkos]

@lilleyse ready

[lilleyse]

Works for me.

[hpinkos]

@lilleyse anything else or is this good to be merged?