Drop Sources/Targets

[helje5]

Noze.io has the concept of streams (GReadableStream, GWritableStream) and generators/sinks (GReadableSourceType, GReadableTargetType). The latter just read and write items while the former provide a buffer and all the event and piping logic.

That separation of concerns seems actually quite good, but also makes the solution quite a bit more complex and adds an extra layer of callback closures.
So in retrospective I think we should rather remove them and just have streams with the relevant logic embedded in subclasses.