Support for ball service; AI improved; gameplay fading

Support for ball service; AI improved; gameplay fading

Add support for doing serving the ball (using a stopped,
non-moving ball that the player has to hit to start the
game) and re-factor ball-resetting code into a seperate
function.

Improve AI player movement to make the game livelier :)

Fade between two scores and block input (both human and
AI input) while the scene is being faded to add a smooth
crossfade between scores. Don't wait for keypresses as
before.

Fade out audience sound when returning to main menu.