Enhance position information stored about each car

[Henrik Heimbuerger]

Currently, the position of each car is just stored as a simple int, representing the step counter.

For future extensions (lane changing in particular), it needs to have the following elements:

• the information which lane the car is on
• the step counter
• the information whether the car is currently on a lane change path
• the information whether the car is on the track in the first place (might have left due to speeding or crashing into another car)

Note that the step counter can change when a car changes from one lane to another, as not all lanes have the same step length.

Part of this ticket is also updating the TrackWidget to support drawing these cases.