Clear tracked object when data sources are removed.
Merge branch 'master' into balloon
Use `defined` instead of `typeof`
Merge branch 'master' into balloon

Conflicts:
	CHANGES.md
Merge branch 'master' into let-go
Use new events to detect when a trackedObject has been removed and fly to home view when it happens.
Merge branch 'let-go' into balloon

Conflicts:
	Source/Widgets/Viewer/viewerDynamicObjectMixin.js
Viewer balloon improvements.

Hide the viewer if the selected object either goes out of it's availability or the data source containing the object is removed.
Merge branch 'balloon' into uberbranch