Use a random number for WebUI cache busting

Also fetch qbt version info via WebAPI properly.

src/webui/webapplication.h

@@ -127,6 +127,7 @@ private:
     Http::Request m_request;
     Http::Environment m_env;
     QMap<QString, QString> m_params;
+    const QString m_cacheID;
 
     const QRegularExpression m_apiPathPattern {(QLatin1String("^/api/v2/(?<scope>[A-Za-z_][A-Za-z_0-9]*)/(?<action>[A-Za-z_][A-Za-z_0-9]*)$"))};