Add "Show unread then starred" option to mails sorting

Checklist

• I have used the search function to see if someone else has already submitted the same feature request.
• I will describe the problem with as much detail as possible.
• This issue only contains a request for one single feature, not multiple (related) features.

App version

6.701

Problem you are trying to solve

IMHO most important messages to be handled are the UNREAD ones, then come the READ BUT STARRED ones. In these 2 categories are mails where some action is still needed, in a basic usage of a mail app.
That is why it could be very useful to keep these 2 categories at hand by displaying them first in the list, before all other read and not starred messages.

Suggested solution

Implement a sort option that has precedence over the other sorting criteria called : "Show unread then starred first".

Screenshots / Drawings / Technical details

Assuming the time is the chosen sorting order, the display result would be with this new option, from top to bottom:

• UNREAD mails, sorted by time (whatever their STARRED statuses are)
• READ BUT STARRED mails, sorted by time
• READ AND NOT STARRED mails, sorted by time

The issue names no files or tests. Start by locating the mail list's existing sorting options and the code that applies sorting, then check how unread and starred states are represented. Done means adding the requested option while preserving the selected secondary time order and verifying the three resulting message groups.