Ubuntu: Enable viewing mailbox subdirectories

Mailbox view can now navigate through child mailboxes.

REVIEW: 117382
BUG: 332989

Fixed translation of Total & Unread message counts

Icon is copied from the ubuntu-mobile-icons package available from
http://packages.ubuntu.com/trusty/all/ubuntu-mobile-icons/download
and is available under the CC-BY-SA 3.0. Copyright by 2004-2012, Canonical Ltd.
Merge remote-tracking branch 'dpniel/mail_box'