FlxAngle: Added angleFromFacing. FlxVelocity: DRY

Accelerate functions now take a single maxSpeed (instead of a maxSpeed separately in x/y)
@Gama11 Not exactly. If you have a separate X and Y you don't have any way of limiting the scalar speed. For example, if max x and y velocity are both 1, your max speed when moving horizontally is 1, but moving diagonally 2^0.5 ≈ 1.41.