Type that implements an interface should not contain additional 'public' members

[RalfKoban]

In case a type implements an single interface - and that interface provides multiple members - we should issue a warning if the type contains other non-overridden members.

The reason behind is following:
The type seems to be the default implementation of that interface.
In case the interface contained some members in previous versions - which have been removed in the current version - the type might not have been updated to reflect these changes. So it still might contain now unused members.

Note:
That rule does not enforce API design guidelines.
The guideline would have been that the interface should be kept unmodified after it has been released.
So the situation that the type contains additional members from previous versions of the interface should not occur.