Clean up viewer use of Balloon

1. Improve GeoJsonDataSource balloon.
2. Make balloon work in all cases in the Viewer.
3. Make the balloon go away when data goes away.
4. Add Balloon.css to widgets.css
5. Misc clean up to viewerDynamicObjectMixin
Merge branch 'hpinkos-balloon' into balloon-tweaks

Conflicts:
	Source/Widgets/Balloon/BalloonViewModel.js
Merge pull request #2 from mramato/balloon-tweaks

Clean up Viewer balloon usage