[Bug-gnubg] Edit mode change proposal

[Philippe Michel]

In edit mode, clicking on the bearoff-side tray clears the board and 
clicking on the midpoint-side one resets it to the initial position. In 
both case the dice and cube are left unchanged.

Wouldn't it make sense to reset the cube to 1 and centered and to clear 
the dice ? If one is going to make such a radical change to the checkers 
setup, it seems consistent to reset the dice and cube to a default value 
as well.

On the other hand, the score or match length wouldn't be reset since 
studying a series of positions at some specific score is a likely use 
case.