Move get_credentials function to the application

[Johannesd3]

I would like to suggest another breaking change before 0.2: removing fn get_credentials. Why? It does not provide any functionality directly concerning Spotify, but it's rather a very specialized function, written with a concrete application/daemon in mind.

Yes, spotifyd uses this function too, or better: they abuse it. Instead of prompting in the prompt callback they abort the application. Which shows that this function is too specialized for their usecase too.

So I would suggest moving it to the application. It's not hard to implement, so every dependency using this function could implement it's own more spezialized function. Do you agree?