Fix delegation of control panel configlet permissions (v4)

This pull-request supersedes #12, the commits have been rebased on master and the new "Themer" role has been factored out into another branch.

At the moment only Managers can access the theming panel provided by plone.app.theming, this couple set of commits fixes that.

DISCLAIMER: I am not a Plone contributor yet, but wanted to put this under review as I needed to have it working at work anyways.