Back the logger with a QFile

Previously all messages were written to a string which grew unbounded
when logging was disabled.

src/WebPageManager.cpp

@@ -7,7 +7,7 @@ WebPageManager::WebPageManager(QObject *parent) : QObject(parent) {
   m_cookieJar = new NetworkCookieJar(this);
   m_success = true;
   m_loggingEnabled = false;
-  m_ignoredOutput = new QString();
+  m_ignoredOutput = new QFile(this);
   m_timeout = -1;
   createPage(this)->setFocus();
 }

src/WebPageManager.h

@@ -52,7 +52,7 @@ class WebPageManager : public QObject {
     QSet<WebPage *> m_started;
     bool m_success;
     bool m_loggingEnabled;
-    QString *m_ignoredOutput;
+    QFile *m_ignoredOutput;
     int m_timeout;
 };