LIB-7: Data tracks

This is a huge change, won't apply cleanly and might need extra work.
This is to keep track about the work done and the work needed.


As far as I understood the whole TOC reading logic is centralized in a platform independent `disc_toc.c` file. Only the actual reading of a disc is platform specific.
Additionally in `disc_unix.c` some of the things that work an all UNIXes are centralized.

In that platform independent `disc_toc.c` [LIB-7](http://tickets.musicbrainz.org/browse/LIB-7) is solved then.