[Feature] Subheadings for subtasks

[skwde]

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

Not having the ability to group subtasks together makes subtasks hard to use when there are many of them.

Describe the solution you'd like

Being able to add multiple named subtasks fields (the field which holds "add a subtask") would be a nice way to support this. An alternative would be to allow subheadings inside the subtasks but then, the problem of sorting arises (https://github.com/TechbeeAT/jtxBoard/issues/150).

Describe alternatives you've considered

https://github.com/TechbeeAT/jtxBoard/issues/150 would also be an option to group them together (without a heading), regardless it would be niche to have subheadings.

Additional context