Improve usability of qmlvideofx example on small touchscreens

Improve usability of qmlvideofx example on small touchscreens

* Add tolerance to mouse press detection on divider and slider grips,
  so that clicks just outside of the grip radius are treated as if the
  click was inside the grip.
* When slider bar outside of grip radius is clicked, increment slider
  position by a fixed amount, towards the click position.

Change-Id: Iefb0b274ee5a97ed5cb789596bf6b3c07a7fc8b8
Reviewed-by: Jonas Rabbe <jonas.rabbe@nokia.com>