Add a new game concept called a `flash'. This is a graphical effect

Add a new game concept called a `flash'. This is a graphical effect
taking non-zero time, which is triggered by the making of a move and
is _not_ hurried to its conclusion by the start of the next move (so
the game redraw function is expected to be able to draw it in
parallel with continuing moves). The only thing that prematurely
terminates a flash is the start of a fresh flash. In particular,
this concept is used to display the completion flash in Net, because
at least _my_ playing instincts make me lock every piece I've
unambiguously placed, and hence the last turn move is instantly
followed by a lock move which was previously suppressing the
completion flash.

[originally from svn r4168]