config change_editor is undiscoverable

At the moment when shelving interactively, the 'edit' function is hidden if you have not defined a change editor.

It would be better if the functionality was shown, and selecting it warned you that you need to configure it. The options I see, are:

Tell the user no change_editor is set and:

1) Prompt them for one to use, and set the variable for them
2) Fall back to the "find_editor" code that we use for commit messages. Not 100% relevant because the editor will be passed 2 files, not just one.
3) Let them know how they can configure it, if they want to do so and come back.