[fontra-workflow] Write an action that trims variable glyphs based on how they are used as components

[justvanrossum]

fontra-workflow is quite capable of trimming axes, aka. slicing the designspace.

However, when we want to export variable components (either as source or as ttf/otf + VARC) it can be beneficial to trim the variable glyphs of redundant axes and sources.

This involves to steps:

1. analyze which slice of a variable glyph's designspace is acgtually used throughout the font
2. slice the variable glyphs accordingly

I'm not yet sure how hard step 1 is for deeply nested components. I assume it should be done in as many steps as there are levels of nesting: process glyphs from shallow to deep usage.

We have the logic for step 2, but it may have to be refactored. It is currently only used for global axes.