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>