langmap: adjust to only color non-unique languages
Fixes #203. * spot/twaalgos/langmap.hh (highlight_languages): Simplify the interface by only taking the automaton to color. * spot/twaalgos/langmap.cc (highlight_languages): Only introduce color for states that have a non-unique language. * tests/core/highlightstate.test: Update and add more tests. * tests/python/langmap.py: Keep the tests simple. * bin/autfilt.cc: Adjust usage and help string.