Do not repeat songs when randomizing

[DeinAlptraum]

Instead of picking a random song whenever R is pressed, this generates a random permutation of all songs, goes to the next one in that order every time R is pressed, and finally generates a new permutation once all songs have come up once. Depending on what your expectations are, I would say this resolves #186

I have only implemented this for the regular randomization (the "in one category" case) since I have no idea what the category feature does or how I can access it...