Move account and mailbox node to server window

The server list view uses custom objects to represent account and
mailbox nodes. These were stored under Accounts, but this has always
been a bit out of place. They are more connected to the view than the
model classes in Accounts. Move them to Applications.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>
Maintain dictionary with subscribed mailboxes

Handle incoming LSUB replies and add the mailbox to a dictionary of
subscribed mailboxes, just like mailboxes are handled. Subscribed
mailboxes have no natural hierarchy, so store the nodes in an array.

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