[wip] new-mail notification observer

[roccoblues]

[This is just a proof-of-concept to see if the general connection works]

Trigger the new-mail logic in a notification observer.

• add a hash to IndexPrivateData to keep track of the notification status per mailbox
• add a new mailbox notification type NT_MAILBOX_NEW_MAIL
• index_mailbox_newmail_observer() listens to NT_MAILBOX events and updates the hash
• handle_new_mail() is called in the index loop and triggers the new-mail logic

We need the new notification because mx_mbox_check() doesn't update Mailbox->has_new like mx_mbox_check_stats(). I still think we should only have one method that does the full check. It would make a lot of things simpler.

I intentionally made this minimally invasive, to just show the concept. I think this can be a first step to simplifying things. For example mutt_mailbox_list() and mutt_mailbox_notify() could be removed.

I didn't polish the message yet. So for multiple mailboxes where one is the current one you get something like New mail =INBOX, =neomutt-devel. But this can easily be changed to New mail in this mailbox and =neomutt-devel.

Also, is a hash the correct data structure? Is the mailbox path guaranteed to be unique?