Add local backups

[yev-kanivets]

Only Dropbox backups are now possible, which makes it's impossible to use without a Dropbox account.

[Quantum-Future]

Yes please, add local backups.
I don't want to use Dropbox, specially with this possible problems: #29

[ezzra]

I just stumbled over this, would really like to have local backup support, otherwise might need to stop using the app :-(

[yev-kanivets]

@ezzra thanks for the input 🙏 The app is in the maintenance mode, so I'm not planning to add any new features for now.