* generator/OpaqueGen.cs: generate a finalizer for classes which
have free or unref methods and ensure it runs on the gui thread.
* glib/Opaque.cs: remove finalize handling.
Fixes a 'resurrection' issue with the previous 419777 fix.
* glib/ManagedValue.cs: rework to ref count a ManagedValue instance
and pass a GCHandle to it around, instead of taking out multiple
gchandles on the managed target itself.
* generator/ReturnValue.cs: use new ListPtrToArray marshaler for
lists with known element types.
* glib/Marshaller.cs: new ListPtrToArray marshaller with more
aggressive list disposal.
* gtk/Container.custom: remove manual Children impl. Use Children
in GetEnumerator instead of pinvoking directly.
* gtk/Gtk.metadata: remove hidden attr and add element type and owned
for Container.GetChildren to generate it properly.