[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