Add designspace path to source UFOs `public.designspaces` lib key

[ryanbugden]

I have just made a proposal for an official lib key in the UFO spec that would point a UFO to all designspaces it’s associated with. If this is standardized, I think it would be great if the designspace-related tools we use keep good care of designspace path links. It is possible that DSE could manage the source UFOs’ libs.

Here's a potential logic below.

If designspace has a path:

• add it to each new source UFO’s public.designspaces lib key. ** If designspace doesn't have a path:
• wait until designspace is saved, then...
• add it to each source UFO’s public.designspaces lib key. **

** If a UFO is open, just add to the key without saving. If it’s not open, write it to the UFO and save.

Reference:

• Designspace Manager, an extension establishing a link within a UFO to all associated designspaces using public.designspaces
• RFont.designspaces, a potential FontParts proposal by @connordavenport