[Development] Foreign windows, embedding and transiency

[Alberto Mardegan]

I got a WIP branch up for preliminary review:

https://codereview.qt-project.org/#change,42633

Any hints (especially on how to write automated tests for the feature) 
are very welcome :-)

I tested it with a simple client and a Gtk socket, and it mostly works 
(still some focusing issues...).

Ciao,
   Alberto