Backup of (standalone) task lists

### Is your feature request related to a problem? Please describe.
After introducing the calendar backup option I hoped that task lists would also be backed up, because some time ago tasks were only able inside of calendars. So I thought today where you can create calendars without tasks and standalone task lists this would be included, but my tasks are not backed up. (Not implemented or bug?)

### Describe the solution you'd like
Please implement an option to backup task lists as well as calendars.

### Describe alternatives you've considered
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### Additional context
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