Make menu_subtree compatible with names

Unique names should be interchangeable with resource ids. However, `menu_subtree()` with a menu structure like the following threw an error:

```erlang
[{page1,[{page1sub,[]}]},
 {page2,[{page2sub,[]}]},
% etc
]
```
This `rid()` fixes the problem; unfortunately I saw no other way than to pass `Context` around.