QmlJS: Share Context.

Previously Context was not entirely thread safe and had to be
copied locally. Now it is thread safe and its lifetime
managed by QSharedPointer.

The non-safe parts were moved into ScopeChain in a previous commit.

Change-Id: I851a93de85cbd6391dbea0fe33b011e2e093addb
Reviewed-on: http://codereview.qt.nokia.com/1695
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@nokia.com>

src/plugins/qmljseditor/qmljseditor.h

@@ -131,7 +131,7 @@ public: // attributes
     QList<QmlJS::DiagnosticMessage> semanticMessages;
 
 private:
-    QSharedPointer<const QmlJS::Interpreter::Context> m_context;
+    QmlJS::Interpreter::ContextPtr m_context;
 
     friend class Internal::SemanticHighlighter;
 };