[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

src/qt/paymentserver.h

@@ -77,10 +77,6 @@ public:
     // Return certificate store
     static X509_STORE* getCertStore() { return certStore; }
 
-    // Constructor registers this on the parent QApplication to
-    // receive QEvent::FileOpen events
-    bool eventFilter(QObject *object, QEvent *event);
-
     // OptionsModel is used for getting proxy settings and display unit
     void setOptionsModel(OptionsModel *optionsModel);
 
@@ -111,6 +107,11 @@ private slots:
     void reportSslErrors(QNetworkReply*, const QList<QSslError> &);
     void handlePaymentACK(const QString& paymentACKMsg);
 
+protected:
+    // Constructor registers this on the parent QApplication to
+    // receive QEvent::FileOpen and QEvent:Drop events
+    bool eventFilter(QObject *object, QEvent *event);
+
 private:
     static bool readPaymentRequest(const QString& filename, PaymentRequestPlus& request);
     bool processPaymentRequest(PaymentRequestPlus& request, SendCoinsRecipient& recipient);