This commit also puts an instance of RemoteTagScanner into the
Instance class, and hooks it into the "add" and "addid" commands.

The TAG_MODIFIED handler (i.e. playlist::TagModified()) works only if
the modified song is the current song - something that is not updated
until SYNC_WITH_PLAYER is finished.  This fixes tag updates right
after a new song is started.

Obsoletes the same variable from AudioConfig.cxx.

Don't use the global variables in class DecoderBridge; instead,
forward these values to the decoder thread via PlayerControl and
DecoderControl.

Move code from replay_gain_get_real_mode().

Replaces the old BoundMethod template.

All remaining events are specific to the Partition.

Move the GlobalEvents::Monitor instance into class Instance.
Eliminate all global variables.

Prepare for moving idle events to class Partition.  Right now, it's
just a wrapper for idle_add().