public.openTypeCategories: add 'unassigned' value

[moyogo]

Besides the four GDEF glyphs classes: class 1 for bases, class 2 for ligatures, class 3 for marks, class 4 for components, there is also class 0 for any glyph not assigned a class value. See Glyph Class Definition Table.
It can be useful to differentiate between glyphs that don't have any specified category in UFO, leaving it to authoring tools to assign a GDEF glyphs class in the font, and glyphs that are specified to be in the unassigned category in UFO, for which authoring tools should not put in any of the classes 1 to 4.

[benkiel]

Agree. Would it be useful to add a note about the different intent of None and Unassigned?

[moyogo]

Agree. Would it be useful to add a note about the different intent of None and Unassigned?

I added the following note:
Note: If a glyph's category is not defined in the dictionary, authoring tools may assign it to any glyph class in the OpenType GDEF Glyph Class Definition Table.

Is that OK?