Fix onDataSourceRemoved, and move onDataSourceChanged.
Viewer can now track which data source has the active clock.
Make "scale" private since it's an implementation detail.
Merge pull request #1442 from AnalyticalGraphicsInc/selection-api

Make "scale" private since it's an implementation detail.
Merge pull request #1436 from AnalyticalGraphicsInc/clock-tracking

Clock tracking
Merge remote-tracking branch 'origin/master' into gltf