Features ipod

This is the updated merge request from Launchpad: https://code.launchpad.net/~daschuer/mixxx/features_ipod/+merge/80743

This branch adds an optional IPod support by compiling Mixxx with libgpod in place and 
scons ipod=1

It supports sorting and searching. 

Currently it acts the same like the banshee feature. While this is already working well and IMHO in a mergable state, the use case is much different, because a media player is a temporary device. 

So we may discuss which features are missing in Mixxx for such for "temporary tracks" on Media players or other removable medias.