Autosave

[keysym]

As tarsila panics on errors, an autosave could be great to avoid losing progress.

Should autosave...

• Overwrite the existing .tarsila OR write to a .tmp/.backup file to recover later in case of a panic?
• Save on a time interval OR based on user events?

[yds12]

Time interval I think would be better, if we decide to save after every edit event it will be too much writing on disc. We can have a const with the interval in milliseconds, so later it can be configurable via config file. I'd prefer it to save to a temp file, it's less intrusive for the user.

We probably need a crate to help finding a cross-platform way of getting a temp directory.

Also creating this ticket to remember to improve our error handling: #33

Let me know if you interested in working on this, so I can mark the ticket as assigned :)