keys in Glyphs userData{UFO.lib{}} should be exported to root of lib.plist

When importing a UFO, Glyphs dumps any lib.plist keys it doesn't recognize into userData{UFO.lib{}}. On manual export it then moves those back out of that subkey in to the root of lib.plist. I've confirmed that this is intentional on Glyphs's part. Examples of such keys:

- public.postscriptNames
- any custom keys (such as org.sil.altLineMetrics, org.sil.lcg.transforms)

write_ufo should follow Glyphs's export behaviour and restore these keys to the root of lib.plist instead of leaving them in a UFO.lib key.

This will also help to avoid the problem where a font ends up with two public.postscriptNames keys, a complete one in UFO.lib and a strangely minimal one in the root.

Start at the write_ufo entry point and inspect how imported userData{UFO.lib{}} values are written to lib.plist. Verify the handling of public.postscriptNames and custom keys such as org.sil.altLineMetrics and org.sil.lcg.transforms; done means these keys are restored at the root without leaving duplicate or nested copies.

Written by the indexing model from the issue text.