glyphsLib should be split into two modules

glyphsLib currently does two very different and orthogonal things:

* provides a Python interface to the Glyphs file format (read/write/manipulate).
* Converts between GS* objects and UFO files.

Obviously I get the idea that people who manipulate .glyphs files will often want to then turn them into UFO files, but (a) not necessarily - there are other things that can be done with Glyphs files! and (b) glyphsLib is not the only source of GS* objects (Glyphs being the obvious other one). Currently glyphsLib is a combination refrigerator/microwave. I would rather have a separate refrigerator and microwave.

Start by mapping the existing glyphsLib interfaces for reading, writing, and manipulating Glyphs files, along with its GS* to UFO conversion paths. Define the boundary between those responsibilities and identify the affected consumers before proposing separate modules; done means both use cases remain available without coupling them.

Written by the indexing model from the issue text.