Add piano keyboard fingering charts

[dspreadbury]

Requested by Alex Plötz:

From this forum post (in combination with the recent addition of fingering charts to SMuFL) I got the idea to include a basic piano key diagram, as is often found in beginner's teaching materials and also for "chord diagrams" in commercial materials for electronic keyboards aimed at the amateur market. It could consist of two basic glyphs for the two parts of the piano octave, and some glyphs of appropriately positioned filled circles to indicate keys to press. Let me know if this is something that you would consider and I will send you a mock-up."