ItemContainer guaranty enforcing

[Xhamolk]

The Basic Functionality for ItemContainer and ContainerItem (637af3ba761ee09686dc9f704c3f3330ef2a7774) has the following flaw(s):

1. It does not guarantee that the _item in use_ is the same item that opened the GUI, so this could lead to exploits for cheating items.

These flaws need to be fixed before I can make a PR against Forge for adding support for this.

[Xhamolk]

Possible solutions for [1]:

• ContainerItem should guarantee that the _item in use_ remains on it's slot no matter what. In my mod, I overrode GuiContainer.handleMouseClick() to stop the interacting with that slot. But it could also close the GUI, just like RP2 does with the canvas bags. However this approach should do it on the ContainerItem.slotClick() method.
• ItemContainer should have another method that checks if the _item being updated_ matches the _item in use_. But this might be problematic for the user to implement. In my mod, I change the damageValue when the GUI is opened on the ItemStack, and change it back when it's closed. I do this so my GuiContainer can track down the item based on it's itemID and the modified damageValue

[Xhamolk]

Solution: Overrode the Container.slotClick() method, so it checks ContainerItem.slotContainsHeldItem(). If that returns true, then prevent the pick up, and call ContainerItem.onPickupPrevented().

I believe this is a bit more safe (gameplay-wise) than throwing the item to the world when it's picked from the GUI, which was the idea I had after observing RP2's Canvas Bags do that.