Remove CesiumViewerWidget

1. Replace all usage of CesiumViewerWidget with the new Viewer widget.
2. Delete `Client CZML` example becasue it's no longer needed and redundant with `Simple CZML Demo`
3. Delete `Two Viewer Widgets` because we aren't ready to support multiple scenes the correct way.  The technique used by this demo resulted in poor performance and lots of data duplication.