Change numeric variation-labels to alpha variation labels

[artasparks]

Suggestion from Anders Kierulf. Go players are typically more familiar with using a,b,c etc. for variation labels than numbers. Numbers usually imply move progressions.

[artasparks]

When this takes place, we might consider adding key-bindings so you can select the labels. If up-and-down keys switch between variations, though, this might be sufficient.

[artasparks]

Is this really a useful feature? I'm not sure. For reference, Eidogo uses numbers GoSWF uses letters WGo.js uses numbers OnlineGo and KGS don't need letters/numbers since they have gametrees.

Backlog for now.

I agree that for diagrams, lower case alphas make mores sense, I'm just not sure if that's true here