GUI: Discard changes button needed

[pjf]

With the application of #1402, refreshing the list of available mods no longer clears the current changeset. As such, it would now make sense to add a "Discard changes" button next to the "Apply changes" button. This should ask if the user if they're sure before actually discarding said changes.

(Presently the user can still discard changes by going to "apply changes" and then hitting "clear", but that's not the most intuitive way of doing things.)