This class was a simple wrapper on top of twa_graph_ptr, but it's
easier to simply use a twa_graph_ptr with a "state-player" property
instead, this way we will be able to modify the automata I/O routines
to support games directly.

* spot/misc/game.cc, spot/misc/game.hh: Rewrite the solver and
pg_printer interface.
* bin/ltlsynt.cc: Adjust.
* NEWS: Mention this change.
* doc/org/concepts.org: Mention the state-player property.