Add InventoryContents#remove method(s)

[MinusKube]

Add some methods to remove items from the inventory, with different methods for:

• Removing the first stack of the found item (#removeFirst(item))
• Removing a certain amount of the found item (#removeAmount(item, amount))
• Removing all the stacks of the found item (#removeAll(item))

These methods would all ask an ItemStack and would ignore the amount property of the given ItemStack to avoid confusions.

We can also make overload of these methods with ClickableItems instead of ItemStacks, but it's maybe not worth it.