Subtle UI change to Mines. Although I mostly find the unified left-

Subtle UI change to Mines. Although I mostly find the unified left-
button interface (same button to open a closed square or to clear
around an open one) to be a massive help, there is one circumstance
in which it frequently kills me: if I click down on an open square I
want to clear around, then the mouse pointer accidentally drifts
over on to the nearest closed square before I release, I'll end up
opening that square instead and (usually) dying. So this checkin
causes Mines to note which type of square I left-clicked on, and to
do nothing if the button release is on the other type.

[originally from svn r6086]