plugins/id3: Add loop on frames to estimate bitrate

plugins/id3: Add loop on frames to estimate bitrate

Estimate the bitrate and length when there's no additional header we
can rely on to get these numbers. This is done by parsing the initial
frames of the file.

This has an impact on performance, since now we are parsing much more
than before. There's a define N_FRAMES_BITRATE_ESTIMATE and
N_FRAMES_CBR_ESTIMATE that need to be tuned in order to obtain the best
compromise between performance and accuracy.