QmlJS: Improve completion of signals.

* Change from Reference to FunctionValue: There was no reason for them
  being References as the argument types never need lookup.
* Enumerate in global completion: When you define 'signal foo(int a)'
  you now get completion for 'foo' and will get the function argument
  hint popup.

Change-Id: Ic348db477a34ba468dfdb690499a9cd8fd605cd2
Reviewed-on: http://codereview.qt-project.org/4774
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@nokia.com>

src/libs/qmljs/qmljsvalueowner.h

@@ -94,6 +94,7 @@ public:
     const ObjectValue *qmlRectObject();
     const ObjectValue *qmlVector3DObject();
 
+    // converts builtin types, such as int, string to a Value
     const Value *defaultValueForBuiltinType(const QString &typeName) const;
 
     // global object