Ot Feature Panel

This is my addition to @Kris Sowersby's excellent OT Feature Panel proposal.

I would suggest adding some sort of on-hover quicklook bubble showing all substitution the feature can do without the need to apply it.

For instance, some features, especially Discretionary Ligatures, often allow user to type a certain string which is then "translated" to a symbol. But most of the times, there's no way for user to know all the available strings.

This could also work for long lists of one-to-one substitutions by previewing just first couple of glyphs in each class.