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/plugins/qmljseditor/qmljscompletionassist.cpp

@@ -156,8 +156,10 @@ private:
         return true;
     }
 
-    virtual bool processSignal(const QString &, const Value *)
+    virtual bool processSignal(const QString &name, const Value *value)
     {
+        if (_globalCompletion)
+            insertProperty(name, value);
         return true;
     }