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.

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