Introduce backtrack limit to depth-first search

  * Depth-first search is given a new limit based on how many backtrack
    steps have been performed. This enables a more flexible use of DFS
    as a lookahead heuristic, where opening the filterWidth would be too
    expensive already.
  * The parameter is also added to several other methods.
  * Breadth-first search is given a new depth limit parameter. This is
    useful for potentially infinite expansion of nodes due to a back and
    forth with an undoing move.
  * A bug has been fixed in parallel DFS with respect to the depth limit.
  * Method signatures have changed, some methods have been renamed.
    All extension methods have been moved to a specific static class.