* glib/UnwrappedObject.cs: New class which holds an IntPtr.
This is used in Value so that we can retrieve the IntPtr itself
for an object property.
* glib/Value.cs: Add UnwrappedObject cast operator.
* glib/Property.cs: If the retrieved value is an object,
and there is no wrapper object, create a new one.
svn path=/trunk/gtk-sharp/; revision=5440
