Expand headers

Problem/Motivation

The headers are reduced for the mail display.
If a user wants to know more, there is no way in displaying more now.

For investigations of mail, this is a common requirement.

Proposed resolution

Add an action link to expand all headers.

As proposed in #2804557: Iterate and render MultipartMessage parts , this expand action could affect all multipart levels and not just the top level mail header.
A full RAW display is still something else IMHO.
We should still remove duplicates such as From / To, because they are already displayed.

Remaining tasks

Define UX / Mock-Up.

User interface changes

An expand link / button in the header area.