Add support for completely transparent and color swatch sprites

[bbb651]

I find it useful to store color swatches in aseprite and this makes them use the compact representation, and I added fully transparent sprites too while I'm at it for completeness

[bbb651]

Unrelated, but I've also looked into the api and there's app.range.slices to get the slices selected with the slice tool, I'm working on cleaning up the script a bit and I'm thinking of adding support for it, do you think it should replace using the selection to simplify the code, or if not which one should have priority incase both exist? It currently only allows rectangle selections because you can't use shift to extend it, so it's functionally the same.

[pancelor]

Looks good, thanks!

re app.range.slices: I'd say ignore it to keep the code simpler; I'm a fan of keeping the code simple and editable rather than stuffing in more features (I already think the normal selection code is maybe too far in the feature-direction)