Explicitly quit when all (relevant) windows are closed

This shouldn't be necessary, works around QTBUG-31569 (Qt5)

Change-Id: Iff157a76a896bc4d6224039ae859788818abba8b
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>

src/app/main.cpp

@@ -505,6 +505,10 @@ int main(int argc, char **argv)
     QObject::connect(&app, SIGNAL(fileOpenRequest(QString)), coreplugin->plugin(),
                      SLOT(fileOpenRequest(QString)));
 
+    // quit when last window (relevant window, see WA_QuitOnClose) is closed
+    // this should actually be the default, but doesn't work in Qt 5
+    // QTBUG-31569
+    QObject::connect(&app, SIGNAL(lastWindowClosed()), &app, SLOT(quit()));
     // shutdown plugin manager on the exit
     QObject::connect(&app, SIGNAL(aboutToQuit()), &pluginManager, SLOT(shutdown()));