More preparations for multi-player support.

This is the beginning of multi-player support.  There will be support
for multiple Partition objects in one MPD process.

When adding or updating a song, we get a log message even if debug is not
enabled. It seems odd that removing a song shouldn't be done at the same log
level; otherwise looking at the log leads you to believe songs are never
removed from the library on update.
Signed-off-by: Dan McGee <dan@archlinux.org>

GMutex/GCond guarantee that the access to removed_song is protected.

Allocate a player_control object where needed, and pass it around.
Each "client" object is associated with a "player_control" instance.

This prepares multi-player support.

This broke sticker and archive support.