`glyphdata.get_glyph` info may not match Glyphs info 100%

For some reason, Cantarell's `caroncomb_dotaccentcomb`, `dieresiscomb_caroncomb`, `dieresiscomb_macroncomb` ,`dotaccentcomb_macroncomb` and `macroncomb_dieresiscomb` are not recognized as `("Mark", "Nonspacing")` but as `(None, None)`.

Several component glyphs, e.g. `_o.numero`, `_slash.zero` and `_slash.zero.osf`, are correctly marked as `(None, None)` by Glyphs, but have actual categories attached to them by `get_data`.

@schriftgestalt Are "private" glyphs, e.g. the ones starting with an underscore, always categorized as `(None, None)`?

I dumped categorizations:
- From Glyphs: [Cantarell_glyph_categories_Glyphs.txt](https://github.com/googlei18n/glyphsLib/files/2849087/Cantarell_glyph_categories_Glyphs.txt)
- From glyphsLib: [Cantarell_glyph_categories_glyphsLib.txt](https://github.com/googlei18n/glyphsLib/files/2849088/Cantarell_glyph_categories_glyphsLib.txt)

(I'm overlooking Glyphs using `"Other"` and glyphsLib using `None` for some sub categories)

Compare the linked Glyphs and glyphsLib categorization dumps, then inspect the glyphdata.get_glyph and get_data entry points mentioned in the report. Determine how composite marks and underscore-prefixed private glyphs should be categorized, and verify that the resulting categories match Glyphs for the listed Cantarell glyphs.

Written by the indexing model from the issue text.