Add option for custom tileobject serialization

[cosmiccoincidence]

Summary

Basically title. This got left out of the tilemap rework but should be considered.

I think @henkhooft may already be on this task.

[Ryan089]

If BroodjeKipcorn is onto this already and this issue is just to track it, that's fine. If he's not, could you clarify specifically what you're after for this issue?

[cosmiccoincidence]

I agree it would be good to clarify it a bit more though I do not know the specifics. John and Broodje were talking about it when John reviewed the Tilemap PR recently.