Recursively check for new messages

When a mailbox is checked for new messages we only check that particular
mailbox. It would be nice if all mailboxes underneath it was checked as
well. Run a recursive check on all mailboxes under the selected mailbox
as well.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>
Pass node as menu item's represented object

We pass wither the account or mailbox as the represented object to the
menu items in the context menu of the server view. We did however
recently switch one of them to pass the node that the mailbox was
attached to since we needed to perform operations on the node. This is
useful in other situations as well.

Switch the represented object for all other menu items to be the node,
making it consistent between all of them.

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