Gladiolus Rag fixes

[jaredjj3]

This PR addresses some issues rendering Gladiolus Rag by Joplin in #78, but does not completely fix it.

• Ensure that the top of a rendering is present.
• Prevent ties from rendering across systems.
• Investigate increasing StaveTie height, part distance, or both.
• Render repeats.

This PR adds basic support for rendering repeat barlines. I'll address alternative endings in lilypond 45b-RepeatWithAlternatives.xml.

Before

After

[rvilarl]

wow! This looks so good!

[rvilarl]

The cross tab elements are missing.

[jaredjj3]

Thanks for calling this out — this part is very tricky. The next step is to make a small vexml integration test case reproducing that.

I have a theory on what's broken.

Beams assume that they will see all their notes before starting another beam. I know this because they are not keyed by anything in their underlying SpannerMap. There's currently no mechanism to have multiple beams open at the same time.

So far, I can think of doing one of the following solutions:

1. Figure out a key for the beam's SpannerMap. Maybe we can infer one from the note's Address. The thing is that I'd have to be very careful with further assumptions about beams (e.g. can beams traverse measure boundaries?).
2. When providing SpannerData (code), include data from all staves. Then, Beam should be responsible for searching other staves for notes that also belong to a given beam opening.

[rvilarl]

I believed not, but...

[jaredjj3]

I'm glad you found this! Music notation is full of wonderful edge cases.