Support sounds and costumes #10
Description
Sounds can have at least two forms: SoundRef which contains a URL, and Sound, which is encoded directly into the file. We may also support things like Midi and ABC at some point.
Costumes can take CostumeRef and Costume which work like sounds, but there should also be a sprite-sheet interaction to bring multiple costumes in directly from a sprite sheet, either by reference or direct encoding.
Direct encoding should be preferred and there should be a way to convert from Ref to encoded.
Support for costumes that are embedded SVG should also be supported, and should be easy to connect to Shimmy.