Set embedded native parent handle on an already created widget window

When the parent already has a window for it (such as if QOpenGLWidget is
used) then the _q_embedded_native_parent_handle property needs to be set
on the window as this will not happen automatically in the platform
plugin.

Change-Id: I7e5816da1c22bf53bb039a19713ae1a3e5970b3c
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>