Show thread mails in a tree view

[romanofski]

Is your feature request related to a problem? Please describe.
Purebred currently shows the mails in a thread as a flat list, but ideally a discussion is a tree. That is constrained by widgets Brick currently gives us.

Describe the solution you'd like
In order to reflect the true nature of email discussions, display the threads as a tree view with the first mail being the branch and the rest of the thread following as leaves from there.

The benefit this brings is quite a chunk of code can be removed since I think we wouldn't need the additional indirection of a thread type any more.

Describe alternatives you've considered
Alternative as a flat list is currently implemented as a way to get going.

Additional context
See mutt as a reference. Personally I hope this is a stepping stone for a full discussion view as it is implemented by sup.

[frasertweedale]

Brick tree widget blog post: https://cs-syd.eu/posts/2019-06-28-microsmos