Changed graphics some more.

- added a start/finish line.
- cars look a bit more like cars.
New track - Magic Garden
WIP: Added event broker.

The event broker decouples event publishers from event subscribers. It is
intended for relative infrequent events, eg, winning a race, crashing out
of a race, etc, so that systems can modify their behaviour accordingly.
Cars slow down when game lost.
Tidied state code.