Add support for disc tag

[WeeGerGai]

The new version brought track sorting to the application, which is great! However, this creates a new issue, in the case of one album or soundtrack having multiple discs (e.g. extras and demos) being disregarded and put next to each other, causing track lists that look like: 1, 1, 2, 2, 3, 3, 4, 5, 6, etc. to be created.

For example, the underlined tracks in the image below are on disc 2, while the rest are on disc 1.

Having the ability to sort by disc, and only then by track would be nice for cases like this.