fix(gui): don't quit application before last event

Quitting the application kills the event loop. So any event past that will not be handled. Probably causes the "BAD!" entries in log.

src/widget/widget.cpp

@@ -583,8 +583,8 @@ void Widget::closeEvent(QCloseEvent *event)
         }
         saveWindowGeometry();
         saveSplitterGeometry();
-        qApp->exit(0);
         QWidget::closeEvent(event);
+        qApp->quit();
     }
 }