Renamed `TaskQuery.by_dossier` to `by_container`.

The Task ListingClass is used by different containers (inbox, yearfolder, dossiers),
so it makes sense to rename the method and move the get_getter in to the query method
itself.
Extend DossierTaskTab tests, to be save that the right query method is used.
Merge pull request #397 from 4teamwork/pg_task_query_by_container

Reworked `TaskQuery.by_dossier`