Handle mailbox status replies

We may receive status replies from the server, indicating things like
the number of messages in a mailbox. Handle replies of that type and
redirect them to the corresponding mailbox object.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>
Show number of messages in server outline view

We recently added support for parsing status replies. It would be nice
if status commands were sent and the replies made the number of messages
show in the outline view next to each mailbox. Add support for sending
status messages, currently manually but that could be made automatic in
the future. Add two columns to the server view, one for the total number
of messages and one for the number of unseen messages in each mailbox.

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