[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 correctly handle the event test
  after the above change (rewrite test_main to allow usage of
  QCoreApplication:: in the tests)
- 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