Swift migration

[Hackmodford]

• Converted Objective-C code to Swift
• Experimenting with using Hive instead of native SQLite db.

fixes #693

[bartekpacia]

Minor but cool (I guess) thing: you might want to use swift-format so that the code formatting is consistent.

Once you install it, just do swift-format format --recursive --in-place . to format all files .swift files in all subdirectories.

I'll look at the code more closely when I find some time :)

[lyio]

@Hackmodford does Hive require migrations?

[Hackmodford]

I think it does. But found Isar works better especially with Flutter Isolates. https://pub.dev/packages/isar

[lyio]

Regarding the Hive part: I understood that the native SQLite database exists so the the plugin can manage its task irrespective of Flutter at all. The Hive DB ist not readily accessible from native code, is it?

[bartekpacia]

Closing in favor of #793. Thanks a lot for all the work anyway, it was useful :)