Allows database plugins to use the EventThread, e.g. for CURL
integration.

Our local EventLoop instance was never run; it was not usable.

Implementations shall use exceptions instead.

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

The old struct config_param remains only for top-level string options.

Since version 2.32, g_thread_init() is deprecated and a no-op.  Let's
upgrade that (optional) dependency so we can remove all those
g_thread_init() calls.

Simplify the definition of many build options.

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

Detach the Song class completely from the public API, only to be used
by SimpleDatabase and the update thread.

Allow database plugins to announce that they have been modified.

[mk: renamed source files, applied coding style, reduced bloat, using
MPD's threading library, using MPD's error reporting and logging
library and refactoring, fixed lots of bugs]

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.