[Qt] misc PaymentServer changes (e.g. changes to eventFilter())

- make eventFilter() private and pass events on to QObject::eventFilter()
  instead of just returning false
- re-work paymentservertest.cpp to directly handle the temp payment
  request file, because of the above change
- delete socket when we were unable to connect in ipcSendCommandLine()
- show a message to the user if we fail to start-up (instead of just a
  debug.log entry)
- misc small comment changes