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.

Obsolete, our logging library doesn't ues GLib anymore.

Move to TagSettings.c and use C99 initializers.

Allow using the Tag.cxx library without the Config library.

Use only ConfigData.hxx in plugin sources to reduce header
dependencies.

Replaces GLib's GError.

Deprecated.

Eliminates explicit global initialisation.