Add support for refcounted types to GBoxed derive macro or add a new macro

[sdroege]

CC @pbor

[sdroege]

CC @elmarco

[pbor]

CC @mbiggio

Initial WIP was here: https://github.com/mbiggio/gtk-rs/commits/wip/biggio/shared-types

Also adding some notes from the chat:

1. we are ok with the SharedType name (this mirrors the presence of Boxed and Shared in the glib_wrapper macro)
2. we are not sure whether we should have a new derive macro or extend #[derive(GBoxed)]

• A new macro keeps things easier
• Extending the existing one makes it more clear that we are still registering a GBoxed from the gtype point of view

[sdroege]

Ideally the derive macro would be able to handle struct Foo(Rc<_>) and struct Foo(Arc<_>) so that the user doesn't have to implement another trait themselves.