dashboard: kill the Qt built-in main window closing mechanism

When the main window is closed, Qt makes QApplication.exec() return, which conflicts with Quamash's implementation of loop.run_until_complete(). The conflict causes Quamash's run_forever() to return earlier than it should, and cause "RuntimeError('Event loop stopped before Future completed.')".

Closes #475

artiq/frontend/artiq_dashboard.py

@@ -59,7 +59,8 @@ class MainWindow(QtWidgets.QMainWindow):
 
         self.exit_request = asyncio.Event()
 
-    def closeEvent(self, *args):
+    def closeEvent(self, event):
+        event.ignore()
         self.exit_request.set()
 
     def save_state(self):