plugins/id3: parse VBR headers

plugins/id3: parse VBR headers

Parse VBR headers: Xing and VBRI. We use the number of frames and stream
size we find in these headers in order to calculate length and bitrate.

We also parse the "Info" header, which is the same as "Xing", but in
this case we assume the file has constant bitrate.

Now the only case in which bitrate is wrong is when the file is VBR but
it has no headers to indicate so.