Ideas:

- Maybe look only in mutable_properties?  Possibly we could then avoid
  the not so nice code we have for the case that there are many users,
  especially ldap users.

- Allow setting all users back to the site default.  This is an option
  since Plone 4.0.  We might as well do this in a backwards
  incompatible way.  We can then lose the zope.app.component import
  unconditionally too.
