#68 Don't destroy m3u on rerip; m3u rebuild

[Phil Stracchino]

This patch fixes bug #532323 by doing two different things.

First, it extensively modifies the .m3u file writing
code so that re-ripping one track on a CD does not
destroy an existing .m3u file. If a pre-existing .m3u
file is found, it will be appended to instead of
overwritten, and Grip will make an intelligent attempt
to determine whether a track is already listed in the
.m3u file before appending it.

Second, it adds an option to completely rebuild the
.m3u file from scratch when rescanning a CD using the
DDJ Scan feature, so in the worst case, if your .m3u
file does get munged somehow (perhaps you changed your
mp3 file format and then re-ripped a track), you can
regenerate the .m3u file by just scanning the disc.

BUGS: If you rip part of a CD, then rip more tracks
from the same CD later, the resultant .m3u file is not
guaranteed to have tracks listed in the correct order.

[Phil Stracchino]

Patch to fix bug #532323