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/libs/qmljs/qmljsbind.cpp

@@ -97,7 +97,7 @@ Interpreter::ObjectValue *Bind::findQmlObject(AST::Node *node) const
 }
 
 bool Bind::usesQmlPrototype(ObjectValue *prototype,
-                            const Context *context) const
+                            const ContextPtr &context) const
 {
     if (!prototype)
         return false;