Raise the changed event in CzmlDataSource when CZML is loaded.

Also clarify the documentation in DataSource to indicate that the changed event should be raised when the name or clock changes.
Add an option to control whether the first data source added to Viewer automatically sets the clock.

Also improve the logic so that changes to the clock afterward, indicated by the DataSource changedEvent, update the clock as well.  Fixes #1134.
Don't fire changed event in CzmlDataSource unless clock or name have actually changed.
Update CHANGES.md
Merge remote-tracking branch 'origin/master' into fixDataSourceChanged

Conflicts:
	CHANGES.md
Merge remote-tracking branch 'origin/master' into fixDataSourceChanged

Conflicts:
	CHANGES.md
Rearrange code to separate the DataSourceDisplay update from unrelated clock setting code.
Merge pull request #1367 from AnalyticalGraphicsInc/fixDataSourceChanged

Raise the changed event in CzmlDataSource when CZML is loaded.