Lua: Add OnDropped function to Things

[Trotim]

Custom objects can now be picked up and dropped. We could use a Lua way to detect this.

This could be a DroppedFunc field in the object cfg similar to UpdateFunc, and/or a OnThingDropped trigger event that returns the Thing (and thus its position).

If dropped on another creature or thing, it ideally is also returned (such as when gold is given to a creature to pay it).

Possible application are single use potions or bombs players can grab and drop to cause magical effects or damage in the area around where it has landed.

[Loobinex]

@Trotim Is the UpdateFunc not already sufficient? That this gets access to when it is dropped?

[PieterVdc]

the trigger #4087 already mentioned, so covered by that issue