make ProFileOption persist during the scan of an entire project

there is no point in throwing away and re-recreating invariant data over
and over ...

shaves off ~15% of the load time of Qt 4.6.

src/plugins/qt4projectmanager/qt4nodes.h

@@ -198,9 +198,6 @@ public:
 
     Qt4ProFileNode *findProFileFor(const QString &string);
 
-    //internal
-    ProFileReader *createProFileReader() const;
-
 public slots:
     void scheduleUpdate();
     void update();