updating rotation center for rotate-drag-rotate

When I drag a rotated shape, the center of rotation doesn't get updated, so when I rotate it again the shape rotates differently. I modified the code so that the center of rotation gets updated during drag. This may not be the desired functionality as in some cases people would want the rotation center to remain in the initial place. Maybe a bool parameter like preserveRotationCenter should be used, or people can save the rotation center coordinates on drag start and restore them on drag end.