Adds a support for subtasks

### Problem

Sometimes a task or a project consists of different sub-steps. The ability to map these sub-steps of a task or project can, in my opinion, support the project by providing clarity, overview and focus.

I wonder, even without Google reference https://github.com/elementary/tasks/issues/213, to what extent Tasks should support subtasks as well. Also for local lists and other backends.

### Proposal

I would like to open my impulse for discussion. In other words, necessity, technical implementation, visual design...

### Prior Art (Optional)

Planner could be a reference here.
https://github.com/alainm23/planner