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>

examples/video/qmlvideofx/qml/qmlvideofx/main-largescreen.qml

@@ -51,6 +51,11 @@ Rectangle {
     property bool perfMonitorsLogging: false
     property bool perfMonitorsVisible: false
 
+    QtObject {
+        id: d
+        property real gripSize: 20
+    }
+
     Rectangle {
         id: inner
         anchors.fill: parent
@@ -65,6 +70,7 @@ Rectangle {
                 right: effectSelectionPanel.left
                 margins: 5
             }
+            gripSize: d.gripSize
             width: 600
             height: 600
         }
@@ -95,6 +101,7 @@ Rectangle {
                 right: effectSelectionPanel.left
                 margins: 20
             }
+            gripSize: d.gripSize
         }
 
         EffectSelectionPanel {