FLEX-34213 - allow adjustment of maxDragRate (patch contributed by 'after24')

Replaced the private static const MAX_DRAG_RATE by the public static property maxDragRate. This allows the value to be manually set to > 30, smoothing the apparent performace of mobile lists.

Note: the patch suggested a default value of 90; I decided to keep the previous default of 30, in order to minimise the chance of side effects.
Signed-off-by: Erik de Bruin <erik@ixsoftware.nl>