Keyboard and mouse controls revised and fixed

First of all, keyboard prompts are now shown properly if you don't have gamepad and/or using keyboard instead. Second, we revised keyboard/mouse controls and changed it to the simplified scheme, same as for gamepad - vertical/horizontal maneuver axes and steering, 3 axes total. You can also steer with mouse, plus mouse buttons work as throttle, left for acceleration, right for braking. Tell us how it works for you!