Synchronization lacks support for merging KDB files

[mstarke]

KDB files do not store UUIDs for Groups. This leads to a duplication of groups on every load when a file need to be synchronised.

A workaround would be to store UUIDs inside a special structure or add them to the Notes for Groups.

[mstarke]

A name based merge method was added in 2a5d301. I have had the idea to write MacPass custom data to transport UUID options across save/loads as well.

[mstarke]

Originally reported here as Macpass adding folders (duplicating itself) #818

[mstarke]

Idea for custom data extension move to #41. Closing this since it's working for all KDB files without the need for custom data.