Added new mail workflow state `mail-state-removed`.

Including transition for removing and restoring a mail.
WIP: Fix tests after renaming.
Add `related_items` method to all documentish objects (documents, mails).

This allows a simple accessor for related_items, without differing between mails and documents.
Add simple helper methods to the mail class, which the document already provides.

This allows a simple handling for all documentish objects, without
differing between mails and documents.
Remover: Added support for mails.