Use forward declaration instead.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Un-inline the methods that use GLib.

Return Path::Null() instead of dying from assertion failure.

Fixes busy loop when USB sound device gets unplugged (Mantis bug
#3824).

Use LockSynchronousCommand() instead of SynchronousCommandLocked().
Fixes regression from commit ef663810 (dead lock due to cond_wait with
unlocked mutex).

This plugin has been defunct ever since Last.fm changed their
protocol.  Since there is no volunteer willing to fix the plugin, I'm
removing it now.