[Maybe] Enforce zones for all masters

[Mark2Mark]

• When adding a zones CP to one master, add it to all other masters
• When deleting a zones CP in one master, notify and delete it from all other masters when confirmed.
• This would require adding some sort of ID to each zones CP, so that their associated other master’s CPs can be identified.

Similar to how Stem values and other properties in the Font Info work now, it makes most sense to have the zones "compatible", unless I can think of plausible reasons why not.

This also fixes a problem with #12 where glyphs can be linked to a zone (which is a glyph-wide connection, not a master-wide connection.

[Mark2Mark]

That's a significant architectural change: syncing CPs across masters requires adding an ID system to zone definitions. This touches parsing, serialization, the CP UI, and add/delete flows.