Speed modifier on Health

This adds a speed modifier when the actors health is above a certain threshold.

For example if you add
```
SpeedOnHealth:
	HPThresholds: 45, 30, 10
	SpeedModOnThresholds: 50, 25, 5
```
to an actor it will fall to 50% speed as soon as its health goes below 45
25% speed when below 30 etc. 
```csharp
[Desc("Changes the speed of an actor when a certain HP loss threshold is exceeded. Requires the two enumerations to have the same corresponding amount of items.")]
```

will document this properly on https://github.com/OpenRA/OpenRA/wiki/Traits