QmlDesigner: use correct icon for vertical slider

Task-number: QTCREATORBUG-9558

Change-Id: Ib06006ea986eea307bfb6fc3289dce7468de0b48
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>