Revert to the old API before commit e9e55b08, removing unnecessary
bloat.

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.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Replaces struct playlist_provider.

To replace libcue, the unmaintained and crashy library.

Add GMutex, GCond attributes which will be used by callers to
conditionally wait on the stream.

Remove the (now-useless) plugin method buffer(), wait on GCond
instead.  Lock the input_stream before each method call.  Do the same
with the playlist plugins.

libcue's track_get_length() returns 0 for the last track, because that
information is not available in the CUE sheet.  This makes MPD quit
playing the last track immediately.  If we set "song.end_ms=0", MPD
will play the track until the end of the song file, which is what we
want.

.. rather then append to the end of the previous one

Cuebreakpoints from the cuetools package has three modes of operation,
and the default is to append pregap (INDEX 00) to the end of the
previous track. This is the behavior most compliant to the existing
cue files.

Here is the patch which fixes the issue. I borrowed bits of
implementation from cuebreakpoints. I assumed that the whole audio
file must be covered by head-to-head going tracks, which is how
hardware CD players probably work. In cue_tag I changed rounding from
rounding up to rounding down because the thing in mpd which calculates
actual track duration (and current position) rounds it down, and I
didn't want to see in my playlist values different from whose in a
now-playing progress bar.

I've compared the resultant mpd behaviour with "mplayer -ss MM:SS.MS"
where the time was supplied by cuebreakpoints and noticed that mplayer
started each track a bit earlier then mpd, though this was the same
before the patch.

This patch adds practical usefulness to the CUE playlist plugin.

The caller should be responsible for building the absolute URI.

This plugin is the groundwork for MPD's future generic CUE sheet
support.  That's not complete yet, e.g. there is no way for a playlist
plugin to address an arbitrary position within a music file.