[Feature] allow second level of sorting for subtasks too

[stragu]

Is your feature request related to a problem? Please describe.

This might be a bit of an off-label use, but I am trying to use a task as a shopping list, with subtasks as items to purchase. I'd like to be able to have the non-completed ones at the top (to buy) and the completed at the bottom, while they are all sorted by Summary next (so I can easily find items to untick again, as there are dozens).

Describe the solution you'd like

I think it could be useful to have the second level of sorting (the "Then order by" section in the Group/Sort tab) also available for subtasks, not just for tasks.

Describe alternatives you've considered

Could not find any. An alternative I thought of would be a text filter to quickly search subtasks.

Additional context

Thanks for a great app!

Your current device and version

• Device: Samsung Galaxy A20e
• Android version: Android 11]
• jtx Board version: 2.15.00.ose
• Downloaded from: F-Droid