Added way to add creation code in rooms and instances

[luizzeroxis]

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Description

Made so the little + icon on creation code in rooms and instances can actually create the script instead of giving an error. Fixes issue #1900. Based on code from #1412.

Caveats

I tried to make it in a way that it could be expanded with other stuff. Maybe you could create derived classes from UndertaleObjectReference instead, so it doesn't clutter it up with a bunch of properties. Also the part that actually creates the code should probably be in the room model, and the part that creates arguments be separated and merged with the one that does that for object events already.

Notes

None