Pass CesiumWidget.clock option in through Viewer constructor.

Also fixed a related bug when multiple animation widgets are in use.  Basically, moving the green speed needle on one widget would break it on the other widget.  The "faster" and "slower" clicks would also then become broken, but the play/pause/rewind buttons would still work (because the buttons don't use the "widgetForDrag" logic that the speed ring uses).

To test the bugfix, and test multiple widgets sharing a clock, use this GitHub Gist of an updated version of [the Two Widgets Sandcastle demo](https://gist.github.com/emackey/b319bbda2fe4f6554362).