Fix PathVisualizer

PathVisualizer was not handling the case when a DataSource is added to the display and it already has entities in it.  While reviewing the other visualizers, I also noticed that Geometry and Path were not unsubscribing from the collectionChanged event.
Add tests for bugs fixed.
Merge pull request #2004 from AnalyticalGraphicsInc/fix-pathVisualizer

Fix PathVisualizer