Custom backup frequency

[nvllz]

Checklist

• [x] I made sure that there are no existing issues - open or closed - to which I could contribute my information.
• [x] I have read the FAQ and my problem isn't listed.
• [x] I have taken the time to fill in all the required details. I understand that the feature request will be dismissed otherwise.
• [x] This issue contains only one feature request.
• [x] I have read and understood the contribution guidelines.

Is your feature request related to a problem? Please describe. It's related to the problem that I have no control over the backup frequency, which seems to be every morning. I want the app to backup the calendar data every x days, but there's no option to set it that way.

Describe the solution you'd like I would like to have a backup frequency setting within the Manage Automatic Backups dialog that specifies the time between each backup call.

Additional context To get more space in the dialog, we could move the "Export events", "Export tasks" and "Export past entries too" checkboxes to another dialog (like the "Manage event types" one).