v0.26.3 Item drops

Mobs can now have random drops. This is implemented for mob spawners.
The format looks like this:

"drop_items": [
        "money",
        "leather_cap"
    ],
    "drop_item_probabilities": [
        0.5,
        0.5
    ]
}

Items must exist in the world 'catalogue' (for now. May allow unique drops later)
To find the item, you must loot remains of whatever creature dropped them. This involves a bit of typing. You can also examine remains first to see what it contains. You can also take remains, but I haven't figured out how to extract the item from them, yet.

I noticed a couple of files I had missed to commit. This might include a compile error that has existed since v0.26.1