[RFE] Additonal information on messages

[rougier]

Note, please see the IDEAS.org file in repository root for existing ideas; maybe
it's already there.

Is your feature request related to a problem? Please describe.
No related problem.

Describe the solution you'd like
For a custom interface ( nano-mu4e) using threads mode, I need to add information in the following way:

For every message, mark them with:

• message is first in message list (:is-first t)
• message is last in message list (:is-last t)
• message is the last in thread (:thread-is-last t)

For each thread root message, mark them with:

• thread root (:thread-is-root t)
• thread count (:thread-count #)
• thread unread count (:thread-unread-count #)
• thread unread first (:thread-unread-first docid)
• thread unread last (:thread-unread-last docid)

This information allows to control more finely how threads are displayed.

Describe alternatives you've considered
I'm currently using the found-handler hook to fill in the information myself using an instrument function

Additional context
See nano-mu4e