SpinBox: add support for selection handles

Task-number: QTBUG-38934
Change-Id: I8f23d20a24977f1787dcd451b48b22326959e3bc
Reviewed-by: Liang Qi <liang.qi@digia.com>
ComboBox: add support for selection handles

Task-number: QTBUG-38934
Change-Id: I0d7bfc821ff345e76d1ae89818f6a5e0120695f5
Reviewed-by: Liang Qi <liang.qi@digia.com>