Mail view: Use zcml instead of grok to register the og customized mail view.

With the changes in `ftw.mail` (see b91f31810bd19f3bd994b960490515d03b162aca)
our customized mail-view template was not used any more.
Mail view: use sprite icons in the atachments list.
Merge pull request #244 from 4teamwork/pg_mail_template

Mail View: reuse the customized template, fix attachment icons
Fixed German translation in logout overlay.
Merge pull request #246 from 4teamwork/pha-i18n-logout

Fixed German translation in logout overlay.
Treeportlet: Implement deferred tree rendering.

When having a large repository-tree with more than 2000 objects,
the three portlet slows down every site reload. To avoid this performance issue
we render the the tree deferred.