Add racing mode

[Fohlen]

It was requested a lot of times. We want to add a official racing mode to the game. It should be:

• time trial (players can connect at an arbitrary time and their best attempt is counted)
• there is a timeout per map (map switch after 5 or 10 minutes)
• adds a new 'start' and 'destination' entity
• add 2 to 3 maps for racing in accordance to our style (metalic)

[Fohlen]

For potential implementations have a look here:

https://github.com/aschaeffer/sauerbomber/blob/master/src/fpsgame/race.h

Please note that this is likewise a breaking change and one needs to increment the major version of TQ.

[arCABAL]

Just going to leave this here for more potential starting points:
https://github.com/andrius4669/sauer-configs/tree/master/racing
https://gitlab.com/t.clastres/racing-xtreme