[feature] mx::api NonTraditionalKey equivalent

[p-groarke]

We've discussed about this in the past, and I've found mixing mx::core with mx::api to be a pain! :)

So, I'm just adding this here in case someday you'd want to add this feature to mx::api.

Cheers

[webern]

If you'd rather I finish up with my branch (I think it's ready) then I can do that instead.

[p-groarke]

Up to you. If you already have the feature up and working + tested, you can just merge that and close my PR. I won't be offended by it ;)