fix: overhaul the index/pager updates

fix: overhaul the index/pager updates

Wherever you are in the index/pager, you will be notified of new mail
when it arrives.

Make sure the last message displayed is the one saying "new mail".

Notes:
    If you have a mail filtering system that marks mail as read, you
    won't get notifications when it arrives in a mailbox.

    When you send an email, the "sent" copy won't trigger a new mail
    notification.

Fixes: #20 - Notification of sent mail
Fixes: #195 - Cursor loses position when leaving pager