Create a separate class to represent the solver of the maze.
Additional search algorithms will be implemented here.
Additional changes:
- Add public methods to mark cells as visited and to get results of
the visits.
- Renamed MazeDirections to MazeDirection.
- Renamed _num_cell_rows to _height.
- Renamed _num_cells_per_row to _width.
- Created a dedicated method to configure a cell's walls in the Maze
class.
- created a dedicated public method to draw a path between two cells in
the Maze class.
