Experimental balance for 2.0

[Alayan]

A branch containing experimental balance changes can be found here :
https://github.com/Alayan-stk-2/stk-cod ... alanceSTK2

It is very much WIP, and the focus is on SuperTux before looking at lower difficulties, but here are the main changes currently:
- Increase the max speed of karts by about 5%
- Make nitro respawn much faster
- Nerf the swatter
- Big nitro gives 2.5 instead of 3.0
- Rebalance of nitro efficiency, lighter karts get a bigger speed boost from nitro than before but they use it faster.
- The bonus duration of a nitro burst (time of full boost without consumption) is reduced by 20%, the minimum burst length is reduced from ~0.534s to 0.5s.
- Increase the turn radius values of all kart classes.
- Make skidding more useful at taking tight curves.
- Reduce the difference between kart classes when "skidding straight"
- Increase the physics FPS
- Yellow skid gives 4.0 speed instead of 4.5
- Red skid is triggered slightly faster

Testing feedback is very welcome.

[Alayan]

The branch now includes much more changes such as :
- Purple skid
- Nitro-Hack (new powerup)
- Electro-Shield (new powerup, graphics WIP)
- New speedometer
- And many many others

[Alayan]

Some video demonstrations for some of the most visible changes.

Purple skidding
https://www.youtube.com/watch?v=M1YvTRQRirs

Nitro-Hack (powerup)
https://www.youtube.com/watch?v=wX181iwNBHM

Mini-Wish (powerup)
https://www.youtube.com/watch?v=YJ9Z2d-RBaU

[fracture]

Electro-Shield (new powerup, graphics WIP)

So basically a reskin of the Bubblegum powerup? Is this going to replace the bubblegum shield or are there going to be two shield powerups in 2.0?