Scene hierarchy expansion fixes

Maintaining expanded paths in PathListingWidget when path signals it has changed but the path value hasn't changed (such as when a ScenePath emits pathChangedSignal() from a plugDirtiedSignal()).

Also reducing the number of updates in the SceneHierarchy by only signalling ScenePath pathChangedSignal() when the childNamesPlug() is dirtied, and not when any others are.