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/qmljshoverhandler.cpp

@@ -269,7 +269,7 @@ void HoverHandler::operateTooltip(TextEditor::ITextEditor *editor, const QPoint
 }
 
 void HoverHandler::prettyPrintTooltip(const QmlJS::Interpreter::Value *value,
-                                      const QmlJS::Interpreter::Context *context)
+                                      const QmlJS::Interpreter::ContextPtr &context)
 {
     if (! value)
         return;