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

The update thread should not affect the rest of the system, therefore
set "idle" priority, and let it only run when nobody else is using the
resources.

This encoding plugin features a fixed-point mp3 encoder,
with faster encoding on architectures without a FPU.

Right now the encoder is limited to stereo and 16 bit depth.
The bitrate and sample rate can be modified in audio_output.

audio_output {
        type            "httpd"
        name            "My shine stream"
        encoder         "shine"
        port            "8000"
        format          "44100:16:2"
        bitrate         "320"  # default: 128
}

Generic approach for the workaround.

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.

Add the TagStream.cxx library, similar to TagFile.cxx, and use it to
load tags from song files inside archives.

Avoid GLib.

Prepare for adding state.

Rename pcm_convert_domain to pcm_domain.  Move it out so we can use it
without depending on the whole PcmConvert library.

I've created an elementary input plugin that plays sound from the
soundcard, so you can use MPD to listen to anything connected to the
line-in jack, or to Video4Linux FM radio cards that send audio through
the soundcard.  There has been a small number of posts here in the
past requesting line-in input, so here is a first, simplistic stab at
it.

The patch adds a new sheme, alsa://, which causes mpd to play data
read directly from a souncdard.  It defaults to hw:0,0, but you can
pass any ALSA device name in the URI.  So, using mpc for example:

 mpc add alsa://
 mpc play

will play from device hw:0,0.

To use a diffferent device:

 mpc add alsa://hw:1,0

Allows building without GLib.  This fails to compile currently,
because GLib is still used in the MPD core.

Prepare for GLib removal.

To replace g_strdup().

The PcmResampler interface is implemented by the two classes
FallbackPcmResampler and LibsampleratePcmResampler.  This prepares for
adding more resampler libraries.