Solve the progressbar not being updated in the last few secs of a track
This is a bit hackish, but no more than in SVN.
On track change, save the current track info to prevtrack_id3. The time
in that struct is updated with the reenabled codec_pcmbuf_position_callback.
We make the WPS aware that it's lagging behind the codec and force it to
refresh when the codec moves to the next track, so that it displays info
from prevtrack_id3. On the PCM track change, it is refreshed again and
prevtrack_id3 is cleared to avoid showing a full progressbar if the user
skips back.