improve support for LTLf semantics
- spot/twaalgos/remprop.cc, spot/twaalgos/remprop.hh (to_finite): New function.
- bin/autfilt.cc (--to-finite): Add it.
- doc/org/tut12.org: Update to use it.
- spot/twa/twagraph.cc (purge_dead_states): Also remove false edges.
- spot/parseaut/parseaut.yy: Do not ignore false self-loops, and add false self-loop on accepting states without successors.
- NEWS: List the above changes.
- tests/core/ltlf.test: New file.
- tests/Makefile.am: Add it.
- tests/core/complete.test: Adjust expected output.