Allow to repopulate the model on dataChanged signal of source model

We store the complete Item object in the ThreadGrouperModel to return
the thread leader or thread descendants on request. However if they change
in the EntityTreeModel we are not notified about that and our internal items
are not updated. As a fast fix we allow to repopulate the model on every
dataChanged signal of our source model. A smarter way would be to updated the
cached items or only store the QModelIndex of the sourceModel and request the
item on the fly.