This fixes the output gliches visible in the previous patches,
where highlighting a state would remove its fill color.
* spot/twaalgos/dot.cc, spot/taalgos/dot.cc: Implement option C(COLOR).
* bin/common_aoutput.cc, doc/org/oaut.org: Document it.
* doc/org/.dir-locals.el.in, doc/org/init.el.in,
python/spot/__init__.py: Use it.
* tests/python/automata-io.ipynb, tests/python/automata.ipynb,
tests/python/highlighting.ipynb: Test it.
* tests/core/readsave.test: Adjust.
* NEWS: Mention recent changes.
