I couldn't find a way to have multiple frames without having to save first and keep re-saving, if the possibility of having frames you could switch between in the program and onion skin would make this perfect. Good work on it so far!
Thanks! Support for layers, frames, onion skins, and backdrops are all planned, but not yet implemented. I hope the app is still useful to you in the meantime. My development focus reacts to feedback, so thanks for letting me know what you're looking for!