complain about missing sources in id-based messages

i considered making this optional on the basis that the source strings
may be assigned externally, but in that scenario lupdate wouldn't be
used to start with.

Task-number: QTBUG-11843
Change-Id: I3f09db902462b8e94f7a134f6fe950562b300307
Reviewed-by: hjk <hjk121@nokiamail.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
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>