Bugfix in MMCoverFlowLayer

- fixed selectedItemFrame to return correct frame
Added stop method to MMFlowViewImageFactory for making operations stoppable
Setting the correct image aspect ratio for visible images
Implemented canceling of pending image loading operations in -coverFlowLayerWillRelayout:
#Refactoring in MMCoverFlowLayoutAttributes

- new method -applyToLayer: in MMCoverFlowLayoutAttributes
- removed -applyAttributes:toContentLayer: from MMCoverFlowLayer