QmlJS: Refactor LookupContext creation for speed.

* If possible, create LookupContexts through SemanticInfo; it caches the
  linked Context and will be faster.
* Contexts now own their Engine.

Reviewed-by: Lasse Holmstedt

src/libs/qmljs/qmljscheck.h

@@ -71,7 +71,6 @@ private:
     Document::Ptr _doc;
     Snapshot _snapshot;
 
-    Interpreter::Engine _engine;
     Interpreter::Context _context;
     Link _link;
     ScopeBuilder _scopeBuilder;