Use non-local solver from pgsolver

Looks like we misinterpreted the meaning of "local" in pgsolver. It
only calculates the correct solution for the root node, but we
need information about other nodes for intermediate
propagation (on-the-fly game solving).

Now we use a non-local solver, which works but is very slow at the
moment because the game has to be constructed anew every time we
propagate. => We should try to keep the game up-to-date as we expand
nodes, if possible.