Repair flashing of moved piece (XB)

Repair flashing of moved piece (XB)

The flashing was not working (at least in GTK), because the redraw of
the square needed even processing to show something. So the draw/erase
loop that does the flashing needs to call DoEvents() to make things
visible. This however could lead to recursive processing of LeftClick,
as the press of a click-click move already enters the move and starts
the flashing, so that the release usually comes before the press
processing has finished. Which would cause it to be interpreted as
a second move identical to the first. A static flag inside LeftClick
now makes it ignore clicks during flashing, which is just what we had
to do with the release anyway. Flashing should be suppressed during
sweep-selection under-promotion.