[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/test/paymentservertests.cpp

@@ -7,12 +7,9 @@
 
 #include <openssl/x509.h>
 #include <openssl/x509_vfy.h>
-#include <QCoreApplication>
-#include <QDebug>
+
 #include <QFileOpenEvent>
 #include <QTemporaryFile>
-#include <QVariant>
-
 
 X509 *parse_b64der_cert(const char* cert_data)
 {
@@ -41,9 +38,14 @@ static SendCoinsRecipient handleRequest(PaymentServer* server, std::vector<unsig
     f.write((const char*)&data[0], data.size());
     f.close();
 
-    // Create a FileOpenEvent and send it directly to the server's event filter:
+    // Create a QObject, install event filter from PaymentServer
+    // and send a file open event to the object
+    QObject object;
+    object.installEventFilter(server);
     QFileOpenEvent event(f.fileName());
-    server->eventFilter(NULL, &event);
+    // If sending the event fails, this will cause sigCatcher to be empty,
+    // which will lead to a test failure anyway.
+    QCoreApplication::sendEvent(&object, &event);
 
     QObject::disconnect(server, SIGNAL(receivedPaymentRequest(SendCoinsRecipient)),
                         &sigCatcher, SLOT(getRecipient(SendCoinsRecipient)));