Combining noteheads, stems and flags from different voices

[joeberkovitz]

The layout proposal in #185 proposed a minimize-noteheads="yes|no" attribute to combine notes of identical pitch in a single rendered glyph, in the specific case where multiple parts are mapped onto a single staff using <voice-layout> or <staff-layout>. The sense was that while the name could be confusing, MNX definitely needs such a thing.

This issue recognizes that need, but goes further to identify this as one of what may be a broader set of engraving use cases:

• combining noteheads from multiple voices in the same part which fall on the same pitch and have the same location and note value. (Note that In such cases, layouts might be neither needed nor used.)
• organizing noteheads of different pitches from multiple voices on a single stem (i.e. chording them) when they share the same note location, value and stem direction

So, not only are the use cases broader (stems as well as noteheads), but they crop up even when multiple parts aren't being "remixed" onto a single staff using layouts.

Responding to this, I propose having new boolean combine-noteheads and combine-stems style properties as per #263. (Let's not get stuck on the names, I just picked one of the alternate suggestions randomly.) Implementing these as style properties lets them govern the rendering of scores even if there are no layouts, by specifying them directly as style props on <global>, <part> or <measure>. At the same time it lets them look just like the originally-proposed layout element attributes from a syntax POV, if one is using layouts.