Refactor SpeedLimit struct

[VictorPhilipp]

The struct is currently located in the TrafficManager.Traffic.Data namespace. This namespace been moved to the TMPE.API project to allow for reuse in other mods.

However, the SpeedLimit struct does not hold any information - it is merely used as a utility class and only consists of static methods which should be avoided (see #211). It also mixes UI and game logic.

Tasks:

• Move all constants (The decision should be based on where the constant is used)
  • either to SpeedLimitManager
  • or to a UI related class (SpeedLimitsTool?)
• Move methods EnumerateSpeedLimits, ToMphPreciseString, ToKmphPreciseString, ToMphRounded, ToMphPrecise, ToKmphPrecise, ToKmphRounded, GetPrevious, GetNext, GetVerticalTextureScale to a UI related class (SpeedLimitsTool?)
• Move NearlyEqual and IsZero to a (new?) utility class, e.g. FloatUtils
• Move IsValidRange to SpeedLimitManager
• Modify VehicleBehaviorManager.FindBestLane() such that the variable optImprovementSpeed stores velocitiy in game units (and not in km/h).
• Modify member variables MinMinSafeSpeedImprovement and MaxMinSafeSpeedImprovement in DynamicLaneSelection such that they hold velocities in game units (and not in km/h).
• If required, add two members velocity (float) and unit (SpeedUnit) to the SpeedLimit struct and update UI code to use it properly (as a composite value). If not required, remove the struct entirely.