FlxAngle: Added angleFromFacing. FlxVelocity: DRY

Accelerate functions now take a single maxSpeed (instead of a maxSpeed separately in x/y)
Because Haxe's switch statement won't let you do this:

```haxe
switch(sth)
{
    case 1 + 1: 2; // error: Case expression must be a constant value or a pattern, not an arbitrary expression
}
```