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().

Use a listener interface instead of GlobalEvents.

Don't use global variable.

Another big chunk of code for multi-player support.

From now on, struct Song will be used by the database only, and
DetachedSong will be used by everybody else.  DetachedSong is easier
to use, but Song has lower overhead.

Fixes regression from commit e96779de.

Finally restores an important feature that has been broken for several
months when the PlayerThread started working with Song copies instead
of pointers to the Queue's Song instances (commit e96779de).

Move code from class Instance.

Remove a GLib dependencies from class EventLoop and DatabaseLock.

This updates the copyright header to all be the same, which is
pretty much an update of where to mail request for a copy of the GPL
and the years of the MPD project. This also puts all committers under
'The Music Player Project' umbrella. These entries should go
individually in the AUTHORS file, for consistancy.

The signal_check library went out of order when we started using the
GLib main loop.  Convert the SIGHUP handler to use event_pipe instead.

Most of these functions were obsoleted when we switched to threaded
MPD.

"LOG_H" is a macro which is also used by ffmpeg/log.h.  This is
ffmpeg's fault, because short macros should be reserved for
applications, but since it's always a good idea to choose prefixed
macro names, even for applications, we are going to do that in MPD.