QmlJS: Reactivate instantiating component scope detection.

* Bind::usesQmlPrototype is now significantly more performant
* type environments are no longer hashed by filename, but rather
  by Document *
* duplicate scope builds are avoided

Task-number: QTCREATORBUG-2835
Reviewed-by: Erik Verbruggen

src/libs/qmljs/qmljsdocument.h

@@ -71,6 +71,8 @@ public:
     AST::ExpressionNode *expression() const;
     AST::Node *ast() const;
 
+    const QmlJS::Engine *engine() const;
+
     QList<DiagnosticMessage> diagnosticMessages() const;
 
     QString source() const;