Beyond composing ligatures, AAT finite-state transducers can also
execute insertion actions without rewriting existing glyphs. The
corresponding actions have almost the same structure as ligature
actions, so we can share a lot of the plumbing within fonttools.
This renaming is in preparation of a larger change to support `morx`
ables with insertions.
Also fixes a bug where glyph alternates in MTI feature files were
wrongly sorted by glyph name. After this change, the output is using
the same ordering as in the input MTI feature file.
Fixes https://github.com/fonttools/fonttools/issues/833.
