Do not remove QtWebEngine/5.x.y from the default user agent to increase compatibility.

Many websites do not recognize the “Falkon” token and show a warning message
claiming the user is using an outdated version of Google Chrome, while they do
correctly recognize known Chromium forks as such.
An example showing this behavior is https://www.whatismybrowser.com/

src/lib/other/useragentmanager.cpp

@@ -28,7 +28,7 @@ UserAgentManager::UserAgentManager(QObject* parent)
     , m_usePerDomainUserAgent(false)
 {
     m_defaultUserAgent = QWebEngineProfile::defaultProfile()->httpUserAgent();
-    m_defaultUserAgent.replace(QRegularExpression(QSL("QtWebEngine/[^\\s]+")), QSL("Falkon/%1").arg(Qz::VERSION));
+    m_defaultUserAgent.replace(QRegularExpression(QSL("(QtWebEngine/[^\\s]+)")), QSL("Falkon/%1 \\1").arg(Qz::VERSION));
 }
 
 void UserAgentManager::loadSettings()