Made StemTremolo into an atom.

Note that there's a small comment included in the parser with this commit.

We'll need a scalable way to determine spanner post events from other post
events. The workaround for now is explicitly to list
post-events-that-used-to-be-marks-but-now-are-just-attached-items. This is the
approach taken in this commit with StemTremolo.
Merged master.