Add a "if holding item" option to loot table options

Blocks with the "leaves" block base dropping with shears doesn't work all the time, and there might be other blocks that we would want to have similar behaviour. I think when making a loot table, each pool should have an item slot, and if you set an item in it, that pool will only be dropped if the player is holding that item. You could also set the other pools to drop only if not holding that item, and there could be a checkbox for whether the item gets damage dealt to it or not. This would make this whole thing so much simpler and give many more options to block drops.