long overdue... this isn't quite as extensible as the original api3/blocks system was, but this is primarily intended to replace Item->useOn(). If plugins want to use it it can be extended later on.
It's only now used in the Durable class, so it does not make sense to keep it in Item anymore. This is a leftover from the days where Durable did not exist.
the count parameter is useless since Item ctor should now only be used for constructing item _types_, not actual items. All item creations for inventories etc, should go through the ItemFactory.
