Reduce piskel model size

Piskel models are now using a base64 png to represent each layer.

Consequently, the dimensions of the piskel are also stored in the model to reconstruct the piskel frames correctly.
The model version has been upgraded to 2.

(This being said, old models can still be loaded by the application thanks to backward compatible deserializers. I don't want to keep doing this for all our model changes, next time, we should do a job to migrate the existing piskels)