Sidebar navigation features

These commits add functionality for switching between mailboxes that have new messages.
In addition to Steve Kemps patch @b2bb0190, I'm adding functionality to iterate though the unread mailboxes
and starting from top if the end is reached.

This is the macro I use in my .muttrc to open the next mailbox automatically:

    macro index <esc>a "<sidebar-new><sidebar-open>"
    macro pager <esc>a "<exit><sidebar-new><sidebar-open>"