Support automatic update even if the app is closed

[llewelld]

Issue #78 and PR #81 introduced automatic daily sync, but the app has to be open for this to work. In a comment on that issue @dimpflmoser made the nice suggestion of using systemd to have the app automatically open, perform the update and then close.

This would be a useful addition to the process. I'm putting it as a separate issue as it needs more investigation and work.

[bionade24]

Could we maybe do this even better by adding a --nogui option to the App? This then triggered by a timer and a notification on finishing would avoid accidental closing of the app.