[Bug-gnubg] Show button on Hint window

[Ian Shaw]

I've just found this feature - very useful.

I think it could be enhanced by only allowing 1 move to be selected when the 
Show button is down.
At present, clicking on the first move selects it and shows the position. So 
far, so good.
The intuitive thing to do is to click on the next position you want to see. 
What happens is that both positions are now selected and the original position 
is shown.
With the Show button down, clicking on a row ought to deselect any previous 
selections, and select the row just clicked.

If you want to go a step further, you could take a leaf out of GammOnLine's 
book. On the published matches, merely hovering the mouse over the move 
selection is sufficient to Show the new position. I find it extremely 
satisfying to use.

--ian