Task operations

[PinewoodPip]

Various task operations need implementing:

• [x] Completing a task. It should also be possible to un-complete a task in case you press the button by accident. For this, I think a Switch UI element would be good - unless there's some built-in checkbox view that I missed.
• [x] Deleting tasks - a button for this already exists in TaskCreator
• [x] Marking tasks as recurring, either weekly or daily. For now, it's only necessary to store this - actual functionality will be added once we start working on the calendar. I think in the UI this could be a spinner for an enum.

Completed tasks should show differently in the TaskExplorer, however we did not define how. I think they should be filtered out by default - but that's for a separate issue.

[PinewoodPip]

Completing tasks now supports recurrent tasks. I would say this part of the project is now feature-complete.