Add basic mouse wheel support to SpinBox

Change-Id: I9305a96d981f19ae85104facf2da01b2b215f316
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Caroline Chao <caroline.chao@digia.com>

src/controls/SpinBox.qml

@@ -192,6 +192,12 @@ Control {
         anchors.fill: parent
         hoverEnabled: true
         onPressed: if (activeFocusOnPress) input.forceActiveFocus()
+        onWheel: {
+            if (wheel.angleDelta.y > 0)
+                __increment();
+            else
+                __decrement();
+        }
     }
 
     TextInput {