Implemented the sort by bitrate option

Hi, I've implement (correctly I hope) the option to sort the songs in the library by bitrate. This is useful to me since sometimes I encode some albums to a lower bitrate (if I want to copy it on an MP3 player) so I have 2 copies of same songs that differ only by the bitrate. So far there was no way (to my knowledge) of separating them in the library, they appeared twice in the "Album" listing and it was a problem to add an album to the playlist since the songs would appear twice. Hope it works fine, I followed the implementation of the "sort by year" case.