Update to Swift 6 and Sendable

[BPerlakiH]

This could improve our concurrency handling and Swift 6 comes with much more restrictions of what kind of data can be sent between threads and what not (Sendable).

This is a rather large change, so needs more preparation, and can be done step by step.

Possible candidates to change:

• Core Data objects should not be sent across threads (they are based on NSObject), such as ZimFile, ZimFileMetaData, etc
• NSPredicates (core data queries) should be better secured, they can be modified on the fly, although we are not using these options, Swift 6 will throw an error on those.