introduce new message type "vanished"

this state is equivalent to "previously finished, but now obsolete", or
in gettext terms "not fuzzy and obsolete".

this bumps the ts format version to 2.1.

Task-number: QTBUG-12948
Change-Id: I6c89bbfb9b16053b4e1219ec7611cb7a34189d95
Reviewed-by: hjk <hjk121@nokiamail.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>