Merge pull request #427 from 4teamwork/lg-hide-duplicate-mail-title

Hide duplicate title in mail preview tab
Add plone.app.lockingbehavior to ftw.mail.mail FTI.
Change mail tabbed view to register tabs with old-style FTI actions

instead of custom TabbedView class.
Add 'Edit Metadata' action to ftw.mail.mail FTI.
Override ftw.mail default view with our customized dexterity default view.