[Recommendation] Add GuiType.ANVIL

[ofirtim]

I think we can agree that anvils are a requirement for the trimuph gui system, now I don't know if there's an existing branch that works on that issue/recommendation, but I I would like to see if there is any possible ways to patch this in, since using Bukkits Inventory management is completely useless once your use this gui lib.

[Azoraqua]

I really would like this as well!