Display the help for various keyboard shortcuts.

def display_help(self):
        """
        Display the help for various keyboard shortcuts.
        """
        helptxt = ["h : Toggle help",
                   "A/a, B/b or C/c : Increase/decrease cell by one a,"
                   " b or c unit vector", "# : Toggle showing of polyhedrons",
                   "-: Toggle showing of bonds", "r : Reset camera direction",
                   "[/]: Decrease or increase poly_radii_tol_factor "
                   "by 0.05. Value = " + str(self.poly_radii_tol_factor),
                   "Up/Down: Rotate view along Up direction by 90 "
                   "clockwise/anticlockwise",
                   "Left/right: Rotate view along camera direction by "
                   "90 clockwise/anticlockwise", "s: Save view to image.png",
                   "o: Orthogonalize structure"]
        self.helptxt_mapper.SetInput("\n".join(helptxt))
        self.helptxt_actor.SetPosition(10, 10)
        self.helptxt_actor.VisibilityOn()