We need to let the QGuiApplication determine whether quitting is appropriate
based on whether there are visible top level QWindows after the last top-level
QWidget was closed.
This solves the issue raised here: http://thread.gmane.org/gmane.comp.lib.qt.user/1880
The transientParent is the QWindow equivalent of parentWidget on QWidget, so the test
in QGuiApplication::shouldQuit is similar to the one in QApplication::shouldQuit.
Change-Id: I500eff8d5887f24415180134b3a4be3c630a896f
Reviewed-by: Bradley T. Hughes <bradley.hughes@nokia.com>
c5665a182d