Fix drag and drop with native widgets

Fix drag and drop with native widgets

Task-number: QTBUG-27336

Don't check 'widget != m_widget' in
QWidgetWindow::handleDragEnterMoveEvent() since the current window's
parent widget may be the actual drop target. I replace it with a check
'!widget->isWindow()' to prevent we pass through a top level window.
I also change 'widget->mapFrom(m_widget, event->pos())' to
'widget->mapFromGlobal(m_widget->mapToGlobal(event->pos()))' since m_widget
may not be widget's parent.

Change-Id: Ia4f10f85ccdf1e27223ddc51afabd98b5d16f2fb
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>