[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/test_main.cpp

@@ -1,8 +1,7 @@
-
-
 #include "paymentservertests.h"
 #include "uritests.h"
 
+#include <QCoreApplication>
 #include <QObject>
 #include <QTest>
 
@@ -11,6 +10,11 @@ int main(int argc, char *argv[])
 {
     bool fInvalid = false;
 
+    // Don't remove this, it's needed to access
+    // QCoreApplication:: in the tests
+    QCoreApplication app(argc, argv);
+    app.setApplicationName("Bitcoin-Qt-test");
+
     URITests test1;
     if (QTest::qExec(&test1) != 0)
         fInvalid = true;