Auto-backup channel file on every Blixt startup.

[hsjoberg]

[Darth-Coin]

Think about to do this task, not every time you open the app, but when you open or close a channel. That is a more realistic and useful backup. Also to contain a quick popup message for the user, when is done and alerting him/her: "hey your backup is ready in xxx location, please make an external copy of that file, in case SHTF with your device".