[kernWriter] Base UFO class kerning on groups
[kernWriter] Base UFO class kerning on groups

This way the UFO kerning classes don't have to follow a specific
pattern, so the kern writer should work out of the box with either
ufo3 or ufo2 type group names.

moyogo/ufo2ft,googlefonts/ufo2ft,jamesgk/ufo2ft,jamesgk/ufo2fdk,googlei18n/ufo2ft