skinWasAltered very slow with many deleted clones

[GarboMuffin]

happens in TurboWarp/extensions#1153 (comment)

_allDrawables just keeps getting longer so skinWasAltered gets slower each time

[fath11]

my idea on how to fix this

create a new list for storing empty drawable/skin IDs

when creating a new drawable/skin, check if there is an empty id, if there is one, use that id, if there is none, create a new item

when deleting a drawable/skin, add its id to the list so we know that the id is empty and available to use