It doesn't make sense for a top-level menu to have an icon, however
we do not have dedicated classes to distinguish these.
Furthermore, the only other place to store an icon is MenuItem.
Storing it there would be highly confusing, as MenuItem-with-Action
then would have two icons: one in Action and one in MenuItem.
And because we need to be able to replace the icon during realization,
this would need to write-through to Action somehow.
That's why I went with Menu, not MenuItem.
This factors out icon realization into its own function, making it possible to
use the same code with other classes that have icon() and set_icon() methods.
This feels a lot more consistent and Unixy:
create_shared_buffer() => shbuf_create()
share_buffer_with() => shbuf_allow_pid()
share_buffer_globally() => shbuf_allow_all()
get_shared_buffer() => shbuf_get()
release_shared_buffer() => shbuf_release()
seal_shared_buffer() => shbuf_seal()
get_shared_buffer_size() => shbuf_get_size()
Also, "shared_buffer_id" is shortened to "shbuf_id" all around.
