ensure correct selection of interfaces

[RGreinacher]

Only necessary interface types are selectable. Check on every service change if the set of connected interface to all projects are necessary. If not, display a warning and deselect the unnecessary interfaces.

• write test to ensure, that a project is not valid if it has an unnecessary interface relation
• rename project model's selected_interfaces() to something like selected_and_necessary_interfaces()
• create a second method called something like currently_selected_interfaces()
• on service's after_commit compare the result sets of selected_and_necessary_interfaces() with currently_selected_interfaces() for all projects (related to this service). If those are not equal:
  • display a warning that Dalphi cutted the relation to unnecessary interfaces and
  • iterate those unnecessary interfaces and derelate them