* src/bin/ltlfilt.cc: Add options --remove-x and --stutter-invariant.
* src/ltlvisit/remove_x.cc, src/ltlvisit/remove_x.hh: New files.
* src/ltlvisit/Makefile.am: Add them.
* src/ltltest/remove_x.test: New file.
* src/ltltest/Makefile.am: Add it.
* NEWS: Mention the new algorithms.