Fixed mail upgradestep `to3401`, when it's run multipletimes.

With the current implementation, the new behavior gets doubled in the fti behaviors list
and makes some troubles when transporting to another clients (see `opengever.ogds.base.transporter`).
Fixed mail upgradestep `to3402`, when it's run multipletimes.

With the current implementation, the classification gets doubled in the fti behaviors list
and makes some troubles when transporting to another clients (see `opengever.ogds.base.transporter`).
Dropped `prevent_deletion` handler.

An event handler for preventing object deletion is the wrong way, furhtermore
it makes impossible to remove repositories for administrators.
Merge pull request #678 from 4teamwork/pg_fix_mail_upgradestep

Fix mail upgradesteps (to3401 and to3402)
Merge pull request #679 from 4teamwork/pg_drop_prevent_deletion_handler

Dropped `prevent_deletion` handler.
Added `relate_to` helper to documents builder, for easy adding relations.
Document: Add `checked_out_by` and `is_checked_out` methods to document class.
Added RemoveConditions, an object which check if a document can be removed.
Added Remover class do handle removing documents.
Added folder_buttons action `remove`.

Including german translations.
Activate remove in dossiers trash tab.
Added direct `css_class` getter for documents on the document class.
Added remove confirmation view, which handles confirmation and deletion it self.
Unify message strings and add german translations.
Add `plonetheme.teamraum` to development packages.

To include the styling for the new remove confirmation view (see branch pg_gever_remove_view).
Updated Changelog.
Added new mail workflow state `mail-state-removed`.

Including transition for removing and restoring a mail.
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.