refined conversion to only react on proper set comprehensions; tuned

moving Pair_inject from legacy and duplicate section to general section, as Pair_inject was considered a duplicate in e8400e31528a by mistake (cf. communication on dev mailing list)

employing a preprocessing conversion that rewrites {(x1, ..., xn). P x1 ... xn} to {(x1, ..., xn) | x1 ... xn. P x1 ... xn} in set_comprehension_pointfree simproc

another Future.shutdown after Future.cancel_groups (cf. 0d4106850eb2);

more robust cancel_now: avoid shooting yourself in the foot;

more robust Session.finish (batch mode): use Goal.finish_futures to exhibit remaining failures of disconnected goal forks (e.g. from unnamed theorems) and Goal.cancel_futures the purge the persistent state;

proper 'oops' to force sequential checking here, and avoid spurious *** Interrupt stemming from crash of forked outer syntax element;

added Output "Detach" button;

skipped proofs appear as "bad" without counting as error;

more method position information, notably finished_pos after end of previous text;