Define more style properties [meta issue]

[adrianholovaty]

This is a meta issue to hold discussion about style properties we should add, now that #263 (and #282) have added styling to the MNX spec. At the moment, we've deliberately only defined one style property, color, in order to keep focused on getting the infrastructure right.

Now that the infrastructure is set up, we should get to work defining other style properties. Here is a relevant snippet from @joeberkovitz's comment on #263:

Here are some possible examples that illustrate the variety of possible styles:

color
stem orientation
note head size
music or text font family
fine-grained X/Y offsets
visibility / audibility
enharmonic spelling variants

What are the must-have styles for MNX 1.0? Let's have a "meta" discussion on this issue, then open separate, individual issues for each agreed-on style property.

To start the discussion, I'll propose "music font family". This seems non-controversial and is definitely table stakes for music styling.

[mdgood]

We might want to broaden the discussion of music font family to more general music and text font properties. There are some interesting design choices there that come up in the larger context.

[clnoel]

In my opinion, the music font is the thing most likely to be ignored when importing a piece of music. Everybody is going to want to use their own font after they finish importing. At Musicnotes, we have two proprietary music fonts and all our pieces are published in those, regardless of what the source was. Text font is slightly more useful, as size, italics, bolding, etc. should be respected even if the exact specified font is not used.

However, I would like to start with something that is more likely to affect the way the imported music is shown, such as stem orientation, stem length, x/y offsets, or note-head size.