Slider: Use forceActiveFocus() instead of focus = true

When activeFocusOnPress is true, activeFocus needs to be set on
Slider instead of the focus.

Change-Id: I73e512a84c3f890a12bea0f78cebd0769077cb6b
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>

src/controls/Slider.qml

@@ -221,8 +221,8 @@ Control {
         drag.maximumX: range.positionAtMaximum
 
         onPressed: {
-            if (activeFocusOnPress)
-                slider.focus = true;
+            if (slider.activeFocusOnPress)
+                slider.forceActiveFocus();
 
             // Clamp the value
             var current = __horizontal ? mouse.x : mouse.y