Store com.schriftgestaltung.fontMasterID in .designspace?

Some of our designers interpolate masters outside Glyphs, which copies the master ID everywhere. When trying to make a Glyphs file again, this leads to an error message.

Might be a broader topic of what could be moved from individual UFOs to Designspace lib keys...

Start by examining how com.schriftgestaltung.fontMasterID is handled when converting between Glyphs files, individual UFOs, and .designspace files. Compare the current placement of master IDs with the reported error when recreating a Glyphs file, then define whether this issue should address that case or the broader question of moving UFO lib keys into the Designspace.

Written by the indexing model from the issue text.