TabBar: keep the current tab within the visible area

This makes it possible for the style to simply increment
or decrement the current index (consider arrow buttons).

Change-Id: I048924d7f1f506ecc3005d2ebdfeb01d5ea6ca40
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>

src/private/TabBar.qml

@@ -114,6 +114,10 @@ FocusScope {
         width: Math.min(availableWidth, count ? contentWidth : availableWidth)
         height: currentItem ? currentItem.height : 0
 
+        highlightMoveDuration: 0
+        currentIndex: tabView.currentIndex
+        onCurrentIndexChanged: tabrow.positionViewAtIndex(currentIndex, ListView.Contain)
+
         moveDisplaced: Transition {
             NumberAnimation {
                 property: "x"