Multiple masters widgets

[davelab6]

defconQt currently has widgets for designing individual UFOv3 fonts. I'm filing this issue to discuss and track development of widgets for designing font families, and how to structure and persist family data structures.

(I recommend creating a milestone for this, to track all issues that grow trufont from a font editor to a font family editor.)

UFOv3 Family Support

Since this is the first UFOv3 editor, how to leverage the new data structures in UFOv3 files for storing font families has not yet been fully explored anywhere (as far as I know.)

Perhaps a single UFOv3 file should be a font family, using conventions and private lib data, where each master and instance in the family is a layer.

Perhaps a UFOv3 file should not be a font family, and there should be a higher-level directory structure for "trufont projects" (or something) that contains a UFOv3 for each master and instance in the family.

Glyph Compatibility

Designing a family requires interpolation, and https://github.com/LettError/mutatorMath is ready for us to begin extending to support Python 3 and UFOv3.

However, before mutatorMath can be used, sets of .glyf files must be compatible, or "prepolated."

This requires many things to match:

• the number of points
• the starting point position
• winding direction
• order of contours and components
• anchor points

Glyphs v2 has a productive UI for this. Here are incompatible glyphs:

Here are compatible glyphs:

Here is a nice dialog for reordering glyph elements:

(images from https://glyphsapp.com/tutorials/multiple-masters-part-2-keeping-your-outlines-compatible)

[adrientetar]

At some point we will need to think about this (for the reasons outlined here, the next milestone will be reducing the number of windows needed to get things done). But it's still far off right now.