This is used by popups for select elements and date pickers among
other things. We can keep using the same RenderWidgetHostViewQtDelegate
implementations for that purpose, and query QPA to decide if they
should be wrapped into their own QQuickWindow.

Longer term, we might want to optionally delegate that functionality
to QML.

Change-Id: I88540ca32a9a707d380dfbf486b7f7806b5b65ff
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>