Set manipulations

In this version, the possibility to use a whole different Set class than the default provided is now available.

For this, a SetInterface was introduced, and as on the meaning of the Set object there was no sense whatsoever to extend the AbstractChange object, this filiation was removed.

The possible BC Break (located on the Set constructor) was avoided, even if passing values other than null, null (which are the defaults) is deprecated and strongly discouraged.