Support 6/9 and other chord symbols with embedded slashes

Currently (1.3 and 2.0), slash characters can only be used to separate the main chord symbol from a bass note. But some chords are commonly written with slashes *within* the main chord symbol. Case in point: C6/9, which many people write instead of C69 or C(6,9).

We can handle this with a very small change to the parser. Right now, the slash is detected very early in Harmony::parseHarmony(), and the chord split in two. We could change this to only reject the split if the portion after the slash does not resolve toa tpc. Or even not splitting if the first character after the slash is a digit. Then the main body of the parser would simply add slash to the list of acceptable separator characters, treating it just like comma. We should also add a rendering rule for 6/9 (at least for the jazz style) to get an appropriate layout, just as we do if you spell if without the slash (69).