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.

* python/spot/impl.i: Add python bindings.
* spot/twaalgos/langmap.cc: Implement algo.
* spot/twaalgos/langmap.hh: Declare algo.
* spot/twaalgos/Makefile.am: Add new files.
* tests/python/langmap.py: Add tests.
* NEWS: Update.