FlxAngle: Added angleFromFacing. FlxVelocity: DRY

Accelerate functions now take a single maxSpeed (instead of a maxSpeed separately in x/y)
First, it is a matter of consistency. Currently `acceleration` is given as scalar and `maxSpeed` is given as vector.
Second, functionality-wise, like what @JoeCreates mentioned, I think most users want to have the speed capped at a scalar maxSpeed regardless of the angle of movement.