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 CL.