Add mailboxes to tree past the second level

We have to traverse down the mailbox tree when we're adding new
mailboxes to it. Part of this process involves extracting the remaining
path from the mailbox in order to find the name of the next mailbox
node. This is done incorrectly by using the substring of the mailbox
parent's path starting from the length of the current mailbox parent's
path. Since the root path is of zero length this will yield zero as
well, and we won't be able to go further down than two levels.

Compute the correct index of where the path continues and use that when
extracting the substring, effectively making it possible to go down more
than two levels.

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