Add check for new messages item to account

When a mailbox is checked from the server view all submailboxes are
checked as well. This is not really appropriate if we want to check that
particular mailbox and nothing else.

Add the menu item to the account as well. If a mailbox is selected then
only check that mailbox. Check all mailboxes if the account is selected.

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