contentWidth has to be updated before contentX as the WheelArea
underneath will clamp its horizontal value to its bounds on change and
thus break the update to contentX if contentWidth is growing.

As contentX is automatically updated by changes to contentWidth it's
generally a good idea to prevent any reactions to  that with a recursion
guard. When updating contentWidth before contentX this becomes necessary
for correct operation.

Task-number: QTCREATORBUG-11699
Change-Id: I34fff7a55e93745d658e8cbb5ac3d430a42770e8
Reviewed-by: Kai Koehne <kai.koehne@digia.com>