Add the deprecated Claim handler functions to Property

Right now these deprecated functions don't fullfil their contract,
as the abstract Entity is not properly implementing them (which is
ok as these function are being phased out).

This is needed in order to actually make the Wikibase code that
still relies on these old functions work with properties (which
is something that used to work in the past when this code was still
in Entity).

Mid-term we will want to remove all usages of these functions and
then phase them out, but for now they should at least be properly
implemented (-> fullfiling their contract).

The code and the tests were copied from item, with only very minor
adjustments.
StatementList has an isEmpty