Only make scrollbars non-interactive on mobile devices.

Before this patch, I could not adjust the scrollbars in a ScrollArea on
a Windows 8.1 desktop computer just because it had a touch screen.

The same problem was observed on Linux (with touch screen).

Task-number: QTBUG-42806
Change-Id: I6f35c40b0362820d7c1685e1b6fc8474432237eb
Reviewed-by: J-P Nurmi <jpnurmi@theqtcompany.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>

src/controls/Private/ScrollBar.qml

@@ -102,7 +102,7 @@ Item {
         onExited: if (!pressed) __panel.activeControl = "none"
         onMouseXChanged: if (!pressed) __panel.activeControl = __panel.hitTest(mouseX, mouseY)
         hoverEnabled: !Settings.hasTouchScreen
-        enabled: !Settings.hasTouchScreen // TODO: touch on desktop?
+        enabled: !Settings.isMobile || !Settings.hasTouchScreen // ### Not ideal, but will usually behave as expected...
         preventStealing: true
         property var pressedX
         property var pressedY