[related glyphs] Add a section glyph-name-based ligatures (using _ to separate ligature parts)

[justvanrossum]

This seems a logical and useful addition to the related glyphs panel.

(We need to make sure that glyph name extensions are handled correctly. Perhaps they should be ignored even.)

Examples:

• f_i ↔ f, i
• ka_virama_saa ↔ ka, virama, saa

[justvanrossum]

Note that this scheme should probably take script glyph name extension into account.

From a user on Discord:

One thing you might want to consider is allowing the user to omit redundant hyphenated script suffixes in ligature names. At least in FontLab (don’t have Glyphs), the glyphs in question are actually named ka-orya virama-orya ssa-orya. It would be convenient if the app could automatically reconstruct a glyph named ka_virama_ssa-orya with a single script suffix, for example.

In Glyphs App, there seem to be glyph names like this: xxx-oriya.