BillboardVisualizer improvements

If a billboard was removed and a new one with the same id readded to a
collection while its events were suspended, the old billboard would not
be properly cleaned up and stay in the scene.
More visualizer cleanup

A continuation of 72c52560b5fd934e941f8c3993974c5fbe847b17 to fix
visualizer behavior when adding/removing entities with the same id while
events are suspended.
Merge remote-tracking branch 'origin/master' into how-did-we-never-notice-this
Merge remote-tracking branch 'origin/master' into how-did-we-never-notice-this
Merge pull request #2223 from AnalyticalGraphicsInc/how-did-we-never-notice-this

More visualizer cleanup