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/qmljsscopebuilder.cpp

@@ -224,7 +224,7 @@ const Value *ScopeBuilder::scopeObjectLookup(AST::UiQualifiedId *id)
 }
 
 
-const ObjectValue *ScopeBuilder::isPropertyChangesObject(const Context *context,
+const ObjectValue *ScopeBuilder::isPropertyChangesObject(const ContextPtr &context,
                                                    const ObjectValue *object)
 {
     PrototypeIterator iter(object, context);