Calculate width of audio formats in ls output.

Calculate width of audio formats in ls output.

Currently the field width for the audio format is hard-coded as three
as the name of each supported audio format (mp3, ogg, aac, wma, spx)
consists of three characters.

However, with the forthcoming flac support, this is no longer true
since "flac" consists of four characters. So the ls command must
calculate the maximal field width of the audio formats of all listed
files.

This is achieved by introducing the new audio_format_width member of
struct ls_widths.