QmlJsCodeModel: Respect QT_NO_CODE_INDEXER environment variable

setting QT_NO_CODE_INDEXER = 1 disables the qmljs code model,
as consequence the qml designer will not work with this setting.

Change-Id: I71a2989b14e55e4d130463edd4a6459dee3cbd3d
Reviewed-by: Daniel Teske <daniel.teske@digia.com>

src/libs/qmljs/qmljsmodelmanagerinterface.h

@@ -285,6 +285,7 @@ private:
     PluginDumper *m_pluginDumper;
 
     QFutureSynchronizer<void> m_synchronizer;
+    bool m_indexerEnabled;
 };
 
 } // namespace QmlJS