special case keepAbove Docks

This can be used by (autohiding) panels to get the panel above other keepAbove windows
Since the activation still impact the stacking order, the panel should still use
QWidget::raise() or eventually KWindowSystem::raiseWindow(WId win) to (forcefully) get the panel on top.

NOTICE: latter is actually a call for tools like pagers and taskbars, don't abuse it but
preferably use QWidget::raise() instead. Unless the user doesn't type into the keepAbove window while
activating the panel at the same time, this *should* be sufficient.


REVIEW: 104279