Fix delegation of control panel configlet permissions

At the moment only Managers can access the theming panel provided by `plone.app.theming`, this small set of commits fixes that.

It also creates a new role for easier delegation of theming to a non-technical person, something that will be very useful now that the needed skills are common among web designers. I have intentionally made this feature completely independent so that it can be safely left out if needed.

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.