Allow sloppy seek gestures

If the touch even is not directly within the Thumb's target area, but
it is within the "slop" range, treat it as a DOWN event on that Thumb.

Otherwise, if the down event is outside of the slop range, it will be
treated as a touch-to-seek event when the UP event happens.
Also pass the X-offset of the left and right thumbs to the index-changed listener.

This allows the listener to perform a visual feedback update to track
the index changes.  E.g., to show a popup above or below the Thumb,
or move a textview containing the left and right values, so that they
are adjacent to their respective thumbs, etc.

If not needed, those callback arguments can be ignored.