Always show the inbox at the top of the list

The tree of mailboxes is sorted alphabetically. The special mailbox
INBOX is often accessed and it would be nice if it was always at the top
of the list. We can do this by adding a special case for the root
mailbox where we locate the mailbox named INBOX and swap it with the
first mailbox.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>