Use correct super-call.
Add inbox null-implementation to fix forwarding autocomplete.

Fixes #578.
Add specific icon and watermark-icons for subtasks.
Merge pull request #590 from 4teamwork/subtask_icon

Add specific icon and watermark-icon for subtasks
Merge pull request #580 from 4teamwork/deif-fix-forwarding-autocomplete

Fix an issue with autocompleting assignee/issuer while creating forwardings.
Add vocabulary that lists all users.