Fix positioning when embedding native widgets such as QGLWidget

Embedding native widgets creates window before ActiveQt has a chance
to set the _q_embedded_native_parent_handle property on the window,
so embedded flag doesn't get set during window creation.
Update embedded flag also when setting window flags.

Task-number: QTBUG-26438
Change-Id: Id29d7123de81d6542dfd6471d6dba3d31bbfe2df
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>

src/plugins/platforms/windows/qwindowswindow.cpp

@@ -1139,6 +1139,7 @@ QWindowsWindow::WindowData QWindowsWindow::setWindowFlags_sys(Qt::WindowFlags wt
 
     WindowData result = m_data;
     result.flags = creationData.flags;
+    result.embedded = creationData.embedded;
     setFlag(FrameDirty);
     return result;
 }