[Feature Request] Support 2Do subtasks creation

Originally posted by @bcutter in https://github.com/nextcloud/tasks/issues/23#issuecomment-587783529

I´m a bit confused on this different behaviour, now reading all your posts understanding why (every app seems to implement an own way of creating a sub-sub-task hierarchy).

My experience:

[1] NC Tasks to 2Do:

1. NC tasks web app: creating root tasks, assigning sub tasks => 2 levels
2. Sync those with 2Do app (iOS & Android)
3. View them in 2Do app: all shown flat as root tasks, hierarchy information is lost.

[2] 2Do to NC Tasks:
The other way around is working great indeed:

1. 2Do app: creating root tasks (type "project" or "checklist"), assigning sub tasks => 2 levels
2. Sync those with NC server
3. View them in NC tasks web app: shown correctly with full hierarchy information

But: the way "[2] 2Do to NC Tasks" is limited to only 2 level hierarchies. Adding sub tasks to sub tasks (3 levels) or more is not possible. Vice versa I can create a lot of levels in NC tasks web app (tested to 5 levels), but those are not synced correctly as described in way "[1] NC Tasks to 2Do".

It´s quite frustrating, having a great server/web app and a great endpoint app seeing both are not really working together. As a workaround I created some "spare" or "template" checklists/projects (tasks able to have child tasks) with the 2Do app, being able to use (rename, move and !!! add sub tasks which permanently stick to their parents !!!) them with the NC tasks web app. Not the best user experience, cause it´s not consistent... :-)

...but the workaround shows: NC tasks app can (and currently does!) SUPPORT those parent-child-connection (I don´t know which technique is used by 2Do app when selecting "project" or "checklist"), but it doesn´t create it. That would be SO powerful!

Start with the issue’s described sync behavior between Nextcloud Tasks and 2Do, and trace where task parent-child relationships are created during synchronization. Done means newly created subtasks can retain their parent relationships in 2Do beyond the current two-level limitation, with the existing hierarchy behavior preserved.