GSoC spanners review/update

[Nathan Chou]

Hello,

I have (except for one question below) finished adapting
Dynamic_engraver and Dynamic_align_engraver to work cross-voice. I
plan to continue with other spanners (perhaps slurs next), but I want
to make sure I am on the right track. I organized my current progress
in the gsoc-2016-spanners branch on my GitHub fork (
https://github.com/starrynte/lilypond/compare/master...gsoc-2016-spanners
); any feedback would be welcome. Should I create an issue and upload
what I currently have (working cross-voice support for dynamics) to
Rietveld for code review? Or should I leave that for the end when the
project is completed?

My other question regards the footnote-spanner acknowledger in
Dynamic_align_engraver, which calls Axis_group_interface::add_element
to add footnote spanners for dynamics to the DynamicLineSpanner. In
what situation is this is actually needed? I removed the
footnote-spanner acknowledger and the reg-tests still passed, and I
couldn't come up with an example where the output differed, even after
adding Footnote_engraver to the voice context.

Thanks again for everyone's help!

Nathan