Change the sidebar to use an array of SBENTRY* instead.  Move the
"is_hidden" into SBENTRY.  Remove the added "prev" pointer from BUFFY.

This way, sorting the sidebar doesn't affect the BUFFY list order, and
we don't need elements inside BUFFY just for the sidebar presentation.

Fix sidebar-next for the case where the mailboxes are unsorted and
$sidebar_new_mail_only is set.  Change sorting not to clump hidden
mailboxes at the bottom, instead simply skip over hidden mailboxes in
sidebar-next/prev.