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.
Fix problem where balloon would temporary appear in the wrong place.

This is kind of a hack and needs to be cleaned up.
Remove use of uniform state from SceneTransforms.wgs84ToWindowCoordinates so that it works before the scene is render (but after the frame is initialized.)
Alleviate balloon arrow jitter.

Still not perfect.
Merge branch 'balloon' into dataSourceBrowser