rose edit: fix duplicate file pages

This allows duplicate file sections to be used in rose edit - for example, a `rose-app.conf` file with:
```
[file:foo(1)]
source=/data/local/bar/spam.txt

[file:foo(2)]
source=/data/local/bar/eggs.txt
```

and a corresponding `rose-meta.conf` file with:
```
[file=foo]
duplicate=true
```

will currently cause more than one entry for the `checksum` and `mode` widgets in their file pages, and will crash when the root page `foo` is clicked on.

The file metadata has been altered so that it does not reapply to duplicate file sections such as `namelist:foo(1)`, and the file page widget will no longer be created for pages like `foo`.

@matthewrmshin, please review.