AnimationAudio: Defer loading of KisPlaybackEngineMLT until image becomes animated.

To save RAM for users who aren't working with animation, MLT and its dependencies aren't loaded until the current image becomes animated.

Because the original design requirements of the new animation audio system didn't account for deferring the loading of MLT until after an animation is loaded some refactoring of the bits and pieces that make up Krita's animation code base were necessary to make things work.