translate: improve handling of Xor and Equiv at top-level for -G -D
* spot/tl/formula.hh: Add variant of formula::is that support 4 arguments. * spot/tl/simplify.hh, spot/tl/simplify.cc: Add option keep_top_xor to preserve Xor and Equiv at the top-level. * spot/twaalgos/translate.cc: Adjust ltl-split to deal with Xor and Equiv for the -D -G case. * NEWS: Mention that. * tests/core/ltl2tgba2.test: Add test case. * tests/python/simstate.py: Adjust expected result.