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. 
`OpenRA` usings need to go after all `System` usings.