public void startScroll(int startX, int startY, int dx, int dy) {
        startScroll(startX, startY, dx, dy, DEFAULT_DURATION);
    }
Start scrolling by providing a starting point and the distance to travel.
The scroll will use the default value of 250 milliseconds for the
duration.

@param startX Starting horizontal scroll offset in pixels. Positive
       numbers will scroll the content to the left.
@param startY Starting vertical scroll offset in pixels. Positive numbers
       will scroll the content up.
@param dx Horizontal distance to travel. Positive numbers will scroll the
       content to the left.
@param dy Vertical distance to travel. Positive numbers will scroll the
       content up.
menudrawer/src/net/simonvt/menudrawer/Scroller.java