Implement fading color effect for led ring deck

[ataffanel]

One classic things to do when using the led ring in shows and demo is to fade from one color to another. This requires to use the solid color effect and to do the fading on the client side which might become impractical when controlling multiple Crazyflie.

If would be more useful to have a fading effect that allows to change the color and set the fading time from the current color to the new one. For more efficiency it would also be good to have the color in one parameter only mapping RGB in 32bit: 0x00RRGGBB.

We should also be able to set the default effect with a compile flag, this way the fading effect can be activated automatically at startup if needed by the application.