translate: add ltl-split option
* spot/twaalgos/translate.cc, spot/twaalgos/translate.hh: Build automata with generic acceptance by doing product of automata for smaller subformulas. * bin/spot-x.cc: Mention ltl-split. * NEWS: Mention the change, and show some results. * tests/core/genltl.test, tests/python/_product_susp.ipynb, tests/python/highlighting.ipynb: Adjust test cases. * doc/org/ltl2tgba.org: Update. * tests/core/gragsa.test: Add another formula to cover more code.