Improve the way transitions are duplicated when preparing the turn-based
game for synthesis. The resulting arena should now be deterministic on
nodes owned by the environment. Also move the code to another file, so
that it is easier to test (e.g. in Python).
* bin/ltlsynt.cc: move the code
* spot/twaalgos/split.cc, spot/twaalgos/split.hh: move the code and
implement the improvements
* tests/Makefile.am, tests/python/split.py: test it
* tests/core/ltlsynt.test: update existing tests to reflect the changes
