This fixes issue #418.

* spot/twa/acc.cc,
spot/twa/acc.hh (acc_cond::acc_code::useless_colors_patterns): New
method to detect patterns of colors allowing other colors to be added
or removed at will.
* spot/twaalgos/cleanacc.cc (simplify_acceptance_here): Use the above
patterns to remove some useless colors from transitions and hope that
this can help simplify the acceptance condition.
* spot/twaalgos/degen.cc (propagate_marks_vector): Use the pattern to
add more colors.
* tests/core/ltl2tgba2.test: Add the test case from issue #418.
* tests/core/ltl2dstar4.test, tests/core/satmin3.test,
tests/core/sccdot.test, tests/core/sim3.test,
tests/python/automata.ipynb, tests/python/decompose.ipynb,
tests/python/merge.py, tests/python/pdegen.py, tests/python/remfin.py,
tests/python/toparity.py, tests/python/tra2tba.py: Adjust all test
cases.
* NEWS: Mention this new feature.