stackUnder does nothing on windows

QWidget::stackUnder does nothing on a window. It should call stackUnder_sys to let the OS change the stacking order of the windows, then send a ZOrderChange event. (In other words, stackUnder should behave the same way QWidget::raise and QWidget::lower do when given a window.)

Also, the MS-Windows implementation of QWidgetPrivate::stackUnder_sys() should use the SWP_NOACTIVATE flag, like raise_sys and lower_sys do, to avoid moving a window under some other windows and then activating it.

Finally, the Mac implementation of QWidgetPrivate::stackUnder_sys() also rejects attempts to change the stacking order of windows. Perhaps it can be changed to operate on windows too. (But even if, on Macs or some other platform, stackUnder can't be implemented for windows, this shouldn't prevent letting stackUnder work on those platforms where it can. The limitation, if any, should just be documented.)