LastFM: Add device scrobbling support (bgo#536389)

Enables Last.fm scrobbling of recent plays from a connected device.
Currently only available on devices supported by the AppleDevice
extension. The new functionality happens upon connection of a device.

Adds a new preference to control the functionality that will be
disabled by default. Also updates the Last.fm help page to explain
how it works.

(Really push the modified files, the previous commit was just
an incomplete changeset that only included a new file.)

Signed-off-by: Andres G. Aragoneses <knocte@gmail.com>