Enable QML to handle widgets.

Enable QML to handle widgets.

Added a qmlwidgethandler plugin that is dynamically loaded if QML
encounters a widget that needs to be included in a hierarchy of QML
objects. This plugin then calls QWidget::setParent to avoid a direct
dependency between QML libs and QtWidgets.

Widgets cannot have Layouts as parents, since layouts are not widgets
themselves. Instead other routines will have to take care of proper parentage.