Syncing between devices

Following up on #76, permit automatically syncing tabsets between devices. I would think that would mean each window would have the union of tabs open on any device, but how to handle deleted windows?
Remove them from all devices? Ah, the joys of networked sync.

Requested by a user via email 2017/10/25.

Start by reading issue #76 and the existing tabset-saving and restoration behavior described by the project. Resolve the synchronization model, including how unions of tabs and deleted windows should be handled, before identifying implementation entry points. Done means the behavior is specified for multiple devices and deletion cases, with verification for those scenarios.

Written by the indexing model from the issue text.