QmlJS: Introduce UnknownValue.

To distinguish known-to-be-undefined from a genuinely unknown value.

Change-Id: I606b4ea4d726f94553400b8950d3c0a4e76564a8
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@nokia.com>

src/libs/qmljs/qmljsscopechain.cpp

@@ -112,6 +112,8 @@ const Value * ScopeChain::lookup(const QString &name, const ObjectValue **foundI
 
     if (foundInScope)
         *foundInScope = 0;
+
+    // we're confident to implement global lookup correctly, so return 'undefined'
     return m_context->valueOwner()->undefinedValue();
 }