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.cpp

@@ -159,6 +159,11 @@ AST::Node *Document::ast() const
     return _ast;
 }
 
+const QmlJS::Engine *Document::engine() const
+{
+    return _engine;
+}
+
 QList<DiagnosticMessage> Document::diagnosticMessages() const
 {
     return _diagnosticMessages;