Very small change of the file structure

I’m right now working to improve the font master UI. (to make it easier to set up variable fonts). That lead to a small simplification of the master properties. There are two new properties `name` and `iconName` the later can be ignored but the name will supersede the `weight`, `width` and `custom` entries. Those will still be there (for now) to not break compatibility with older versions. But if you find a `fontMaster.name`, you should use its value instead of the other three.

Start by tracing how glyphsLib reads and converts fontMaster properties from .glyphs files. Verify where weight, width, and custom are handled, then determine how to prefer fontMaster.name while preserving the legacy properties for compatibility. Done means font masters with name use it without breaking older files.

Written by the indexing model from the issue text.