Make GlyphsBackend writable

[ollimeier]

Fixes #75

I added all missing 'put' methods from WritableFontBackend as stubs.

This is a draft PR for getting feedback early.

[justvanrossum]

All good so far 👍

[ollimeier]

@justvanrossum
As you said, I compared the fontra backend and the designspace backend. Therefore there are two options (also written in the code):

1. option, similar to fontra backend: Convert the fontra glyph and replace the glyph in the rawGlyphsData and save it via openstep_plist.dump().
2. option, similar to designspace backend: Instead of using fonttools for writing UFO use glyphsLib for writing to a GlyphsApp file

Would be great if we could talk about it tomorrow: c4efd49

[ollimeier]

@justvanrossum So, the whole formatting with openstep_plist sucks even more. It's not just about formatting, even some parts are reordered, like in the following example the order of the kerning information:

This is the reason why I implemented _findAndReplaceGlyph. This method finds the glyph 'chunk' via start and end row and replaces only this part, without openstep_plist. This is how a diff looks like then (I changed the name for every glyph, so we see a difference):

Via glyphsLib.writer.Writer we get the right formatted text for GlyphsApp 2 or 3. Replacing a whole glyph of a glyphspackage is also very easy and very close to what we do in the fontra backend:

    def _findAndReplaceGlyph(self, glyphName, f):
        filePath = self.getGlyphFilePath(glyphName)
        filePath.write_text(f.getvalue(), encoding="utf=8")

I think this might be better and is probably closer to your original idea?