Sidebar initial sorting support

Hi,

I've implemented a initial sorting support for the sidebar, which BTW I find a very attractive feature for mutt. I implemented it because when working with my IMAP accounts, the sidebar was showing the folders as they were reported by the server, and not how one is used to get them displayed.

While testing this, I also found a strange behavior in how the "mailboxes" command is generated for IMAP folders. Please see the commit messages for details.

Please let me know if you think the implementation is good enough.

Cheers,

Rodrigo