Feature request: Replace art on cloned token

[BadIdeasBureau]

Description: When a token is cloned, instead of the clone inheriting the token art from its parent, replace its art with a specified file.

Potentially useful for encounters where you want to mirror a token's movement with something supernatural (e.g. this discussion here: https://discord.com/channels/170995199584108546/513918036919713802/798212131530735626), or for presenting tokens in a cloned region as "unknown entities" (e.g. for tokens moving within darkness or fog)

[DFurnes]

I needed the same thing for an upcoming session. Took a stab at implementing it in #61.