Scheme inheritance: define schemes in terms of other schemes, to factor out commonalities

The idea is to allow a scheme (permission, notification etc) to be defined in terms of another scheme, and extend it in some direction. Changes to the underlying scheme would take effect in all 'extending' schemes immediately.

> More important, however, whould be the possibility to let a project "inherit" the permission scheme of its super project (if there is a super project) and to let a super project inherit the permission scheme of a project group (if there is such a group). So, we're actually looking for permission scheme inheritance in the project group/super project/subproject hierarchy. Of course it should be possible to override the inheritance and specify a custom scheme anyway.