argument.  Consider all possible combinations of propositions when
generating arcs.  Suggested by Jean-Michel Couvreur.
* src/tgbaalgos/ltl2tgba_fm.hh (ltl_to_tgba_fm): Adjust.
* src/tgbatest/ltl2tgba.cc: Honor -fx.
* src/tgbatest/spotlbtt.test: Exercise -fx.
* wrap/python/cgi/ltl2tgba.in: Support Couvreur/FM with exploded
properties.