Add additional methods to Item

This PR adds methods for manipulating the pickup delay and despawn time of an Item.

Vanilla has a concept of an owner, which can be set through vanilla commands as the `Owner` NBT tag. Only an owner can pick up an item, **except** until 10 seconds before the item despawns. Then, anyone can pick it up.

The concept of an owner wasn't added, as this can be done through events. However, I'm concerned that this might cause plugins to interact badly with command blocks, which might set an owner. This would be opaque to plugins
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