Added YearFolder builder.
Added `client_id` indexer for docuemnts inside an inbox.
Added Upgradestep wich reindexes `client_id` of all inbox documents.

See previous commit for more informations.
Inbox overview tests: cleanup and fix inheriting issue.
Inbox overview document box: show only documents from the current OrgUnit.

See bba522f6c192ed13b923690f305b0e6b7ce13ee1 for more information.
Moved inbox tabs (documents, trash) into a separate tabs file.
Inbox documents tab: show only documents from the current OrgUnit.

See bba522f6c192ed13b923690f305b0e6b7ce13ee1 for more information.
Inbox trash tab : show only documents from the current OrgUnit.

See bba522f6c192ed13b923690f305b0e6b7ce13ee1 for more information.
Rework Inbox tasks listings.

 - Implements as a GlobalIndexTask Listing.
 - Shows only task assigned to teh current orgunit.
 - Use Task.query instead of the query utility.
TaskBuilder: Make sure SQLTasks are updated when change state or set modification_date.
InboxOverview: Rework task boxes:

 - Queries the GlobalIndex for all boxes
 - Shows only task assigned to the current orgunit.
 - Use Task.query instead of the query utility.
Inbox Tabbedview: suffix the title with the the current orgunit's label.
Merge pull request #287 from 4teamwork/pg_inbox_rework

Inbox rework (rework mandanten concept)