Parse the R128_TRACK_GAIN comment string.

Call decoder_timestamp().  This is not necessary currently, but will
be as soon as we implement seeking.

This requires seeking to the end-of-stream, checking its granulepos,
and then seeking back to the previous file position.  We do this only
for local files.

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

Reduce heap allocator overhead.

Replaces GLib's GError.

Avoid duplicating the tag.

Would require gcc 4.7, and MPD attempts to be compatible with gcc 4.6.

Hide the definition from C code, to prepare the transition to C++.