The "pure" and "const" attributes are not so well-defined, and a
recent clang version implements an optimization which pushes the
definition's boundary beyond what I believed it was.  clang now
assumes that functions declared "pure" cannot throw exceptions, even
if they lack the "noexcept" specification.

When compiled with this new clang version, MPD will crash randomly if
an exception happens to get thrown by such as "pure" function
(https://github.com/MusicPlayerDaemon/MPD/issues/41).

This commit removes all such misplaced "pure" and "const" attributes,
closing #41.

Replaces the old BoundMethod template.

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

All remaining events are specific to the Partition.

Use MaskMonitor to eliminate duplicate code.

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

Keep only a reference.

Replaces global variable "main_loop".

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

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

Move global variables into the new classes.  That may allow multiple
update threads for multiple databases one day.

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.

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).

More preparations for multi-player support.