See commit 71f0ed8b

With this commit, multi-player support becomes possible... it's just
not wired to the frontend yet.

This is based on massive amounts of refactoring work I did over the
past 9 years.

There's no point in passing a LightSong reference here; the callee is
interested only in the URI.

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

Decouples db/update/Remove.cpp from global variables.

Move from db/DatabaseGlue.cxx, eliminating global variable.

Don't use global variable.

This commit adds the NeighborPlugin API which can be used to detect
nearby file servers that can be used by input plugins.  This list of
servers is exported using the new "listneighbors" command.  The idle
even "neighbor" notifies interested clients when a new neighbor is
found or an existing one is lost.

There's a lot missing currently: protocol&user documentation, and a
way to "mount" remote servers into the music database.  Obviously,
some code from the UPnP database plugin can be moved to a neighbor
plugin.

Allow database plugins to announce that they have been modified.

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.

Move code from class Instance.

More preparations for multi-player support.

Allocate a decoder_control object where needed, and pass it around.
This will allow more than one decoder thread one day.