Add support for a conversion queue

Normally, I usually convert a unique ISO file from time to time. But I recognize it's very tedious to convert many images one by one. I'm not really interested in a full game collection management, because there are many alternatives out there far more used and mature.

However, some kind of basic conversion queue would be something really useful. My intention is to keep Yawf as simple as possible, and I'm not very sure how to adapt the UI and the interaction pattern to handle this, but something like the play queue present in Totem is very similar in what I'm thinking on.