QmlJS: Allow lookups on const Contexts. Pass const Contexts where ok.

In preparation for caching Contexts.

Reviewed-by: Erik Verbruggen

src/libs/qmljs/qmljsevaluate.cpp

@@ -36,7 +36,7 @@
 using namespace QmlJS;
 using namespace QmlJS::Interpreter;
 
-Evaluate::Evaluate(Context *context)
+Evaluate::Evaluate(const Context *context)
     : _engine(context->engine()),
       _context(context),
       _scope(_engine->globalObject()),