Adds additional QStandardItemModel compatibility.

This pull request will add the appropriate QStandardItemModel calls when rebuilding the model.  beginResetModel and endResetModel will inform any views to wait to update until the model has been rebuilt.  These are also necessary for any proxy models that may be using a ShotgunModel as a source.

I also set to block signals while the rebuild is happening so signals aren't being rapidly sent during a full refresh.

Thanks!