Only quit if there are no visible widgets or windows.

Only quit if there are no visible widgets or windows.

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>