Refactor to represent download state using Flow

[PrimoDev23]

Hey, I just decided to give this a try, so feel free to apply that PR to your library. This refactoring solves some issues like memory leaks that might occur when injecting an instance of Ketch. Also I removed the DownloadManager as a state holder, since the user of the library should better take care of state handling and freeing resources.

This way the library also follows a more reactive approach. Feel free to comment on it.