scrobbler: Change Scrobbler owner to be JuK, not PlayerManager.

PlayerManager will always emit a track changed signal, which means the
Scrobbler will always have to receive it even if we're not scrobbling
(my last change to avoid making network requests when not using last.fm
broke this, it seems).

Instead move the Scrobbler into the base application class where we can
track whether scrobbling is enabled or not, and Do the Right Thing. I
don't have a last.fm account so although this should still work (I
think), I can't verify. I have verified that we at least don't crash
anymore. Martin, can you double-check when you get a chance?

FIXED-IN:4.11.2