More ScrollPane cleanup

ScrollPane and ScrollBar now use minimal event dispatchers.

Set gestures on the ScrollPane _XOR_ the Scrollpane's content container.
Supported content gestures are drag and scale, both of which are handled
programmatically by ScrollPane (i.e. no nativeTransform on the content
container). IF gestures are defined for the content container,
nativeTransform is disabled on the ScrollPane.

See the example code in gestureworks-flash-examples.