GUI updates, error messages, custom quality settings, MLG.tv support

I've changed layout a bit, which is a lot better in my opinion. I've also added MLG.tv support, support for custom quality settings, and error messages if livestreamer fails (when its return code isn't 0). Basically, I've made it a bit more usable for myself.

There is a side effect of the last change: livestreamer is now closed when the GUI is closed. It would be nice to detach the livestreamer process when GUI is closed, but I don't think Qt 4 allows to do that.