imap: fixed buggy names in generated URLs

For some reason the ImapUser variable gets emptied every one mailbox that gets
parsed. This produces the generated URLs to be not consistent among them,
generating, for example, "imaps://host.com/FolderA" and then
"imaps://user@host.com/FolderB". I haven't find the root cause of the problem,
but this at least fixes the issue temporarely
  sidebar: added initial sorting support

The initial sorting support sorts the mailboxes by hierarchy/path. This is a
nice feature to have when a user, like me, uses mutt to check its imaps
accounts.

The sorting method is still fixed, but it could be extended using a new user
preference
Don't consider TopBuffy as someone special, sort it as all the rest
Signed-off-by: Rodrigo Javier Tobar Carrizo <rtobar@csrg.inf.utfsm.cl>
Merge branch 'master' of https://github.com/rtobar/mutt-kz
sidebar: extend sidebar sort methods, add sort_sidebar=

 sort_sidebar=(desc|path|count|new|unsorted)

Signed-off-by: Karel Zak <kzak@redhat.com>