qDBusInterfaceFromMetaObject: avoid quadratic complexity.

Don't use QString::prepend() in the loop. Just compose
temp string by appending, and then prepend whole temp string.

Change-Id: I6efb2d20e03f6a3526103d3a9494d5d1b0fbbf81
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
Reviewed-by: Edward Welbourne <edward.welbourne@theqtcompany.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/dbus/qdbusmisc.cpp

@@ -99,9 +99,14 @@ QString qDBusInterfaceFromMetaObject(const QMetaObject *mo)
             if (domainName.isEmpty()) {
                  interface.prepend(QLatin1String("local."));
             } else {
-                interface.reserve(interface.size() + organizationDomain.size());
-                for (const QStringRef &x : domainName)
-                    interface.prepend(QLatin1Char('.')).prepend(x);
+                QString composedDomain;
+                // + 1 for additional dot, e.g. organizationDomain equals "example.com",
+                // then composedDomain will be equal "com.example."
+                composedDomain.reserve(organizationDomain.size() + 1);
+                for (auto it = domainName.rbegin(), end = domainName.rend(); it != end; ++it)
+                    composedDomain += *it + QLatin1Char('.');
+
+                interface.prepend(composedDomain);
             }
          }
      }