Splitting MNX-Common and MNX-Generic

Hi all,

In defining the MNX format, we've envisioned having a single format that
contains at least two "types" of music notation data — MNX-Generic and
MNX-Common. I've come to believe this isn't the right move.

(For background, see the current spec at
https://w3c.github.io/mnx/specification/)

I believe MNX-Generic and MNX-Common are sufficiently different that they
should just be two separate formats, rather than us trying to shoehorn them
into a single format. Three reasons:

1. It's confusing for end users. Which one should users choose when
importing/exporting? How would the user understand the differences in the
level of semantics between the two similarly named formats? In my own
experience, it's already confusing enough explaining the difference between
a PDF and MusicXML, and those have different names!

2. It's confusing for developers. The type of development work required to
parse something that is essentially an SVG wrapper versus a semantic
document model is completely different. I would estimate 90% of developers
would only want/need to parse one or the other — which strongly suggests
they should be separate formats.

3. There's no obvious benefit in combining them, apart from some negligible
things like having a common metadata format.

With this in mind, I'm proposing we formally split this into two formats,
with distinct names and distinct specs. (To be clear, I think there's clear
value in both formats existing.)

I brought this up with the other co-chairs in our latest meeting — you may
have already seen the meeting minutes — and we're in broad agreement. Here
are the meeting minutes with more details:
https://www.w3.org/community/music-notation/2019/03/05/co-chair-meeting-minutes-march-5-2019/

Can anybody articulate a reason why we shouldn't make this change? I'd like
to make sure we aren't overlooking a benefit that a single format would
give us. If not, I can begin the work of splitting the spec in two. Please
make any comments on the relevant GitHub issue (
https://github.com/w3c/mnx/issues/98), so that we keep all the feedback in
one place.

Cheers,
Adrian