Holds down the left mouse button on an element.

        :Args:
         - on_element: The element to mouse down.
           If None, clicks on current mouse position.

def click_and_hold(self, on_element=None):
        """
        Holds down the left mouse button on an element.

        :Args:
         - on_element: The element to mouse down.
           If None, clicks on current mouse position.
        """
        if on_element:
            self.move_to_element(on_element)
        if self._driver.w3c:
            self.w3c_actions.pointer_action.click_and_hold()
            self.w3c_actions.key_action.pause()
        else:
            self._actions.append(lambda: self._driver.execute(
                                 Command.MOUSE_DOWN, {}))
        return self