Add "single/short beat" conductor symbol

[hrnbouma]

This example is from Steve Reich, "Tehillim", where the triangles and vertical lines indicate long and short beats respectively. Of course there are other "vertical line" characters that could substitute, but at least in Bravura none of them match the line weight and rounded stroke ends of the other conductor symbols, as well as being scaled differently.

[dspreadbury]

This glyph will be called conductorSingleBeat and have code point U+E89B.