Bugfix: correctly manage the AlbumModel when:
* A second sort request is made when a first one is already being processed. The implemented behavior in this commit is to cancel the previous process and initiate a new one, with the new sorting order.
* A filter is requested when the model is being sorted. Previously, this led to the model trying to set the 'show' column to albums that weren't currently in the tree model, given that they were removed to reorder them, and thus causing a lot of "invalid tree iter" assert fails. The current behavior is to check if a tree iter is valid before trying to asign the 'show' value.