QmlJS: std::set instead of QList

This reduces the CPU time of Export::visibleInVContext() from 50% to 15%
on Windows, when switching files.

Change-Id: Iff82924c47d1b696c9d3d7ca40f49d9d02bcb3e6
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>

src/plugins/qmljstools/qmljsmodelmanager.cpp

@@ -281,7 +281,7 @@ void ModelManager::delayedInitialization()
 
     ViewerContext qbsVContext;
     qbsVContext.language = Dialect::QmlQbs;
-    qbsVContext.paths.append(ICore::resourcePath("qbs"));
+    qbsVContext.paths.insert(ICore::resourcePath("qbs"));
     setDefaultVContext(qbsVContext);
 }