Added api function to specify speed factor of an animation

Added api function to specify speed factor of an animation

The speed factor is a multiplier of the normal velocity of the animation.
Values between [0-1] will slow down the animation and values above 1 will speed
it up. It is also possible to specify a negative multiplier so the animation
will play in reverse.

Change-Id: I4e74cb3cd5372f4c141089a05be8325e0892ce7f