isOpenable for InventoryHolder

Please add a method to check if a certain InventoryHolder's Inventory can be opened because there are several hard-to-check-for ways that this could be blocked:

• Some materials above chests/enderchests allow for opening, some don't
• A cat sitting on top of a chest
• Double chest handling is even more complicated in those cases.
• Some InventoryHolders don't have such requirements at all (like furnaces)

Therefore an isOpenable method that checks on the serverside if a player would be able to open a certain container would be very helpful and makes code much more elegant as lots of special cases could be avoided.