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.
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.
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 e96779d).
