GNUstep compatibility: stop using the API feature isFlipped/setFlipped

GNUstep compatibility: stop using the API feature isFlipped/setFlipped
to translate y-coordinates from the default of up-from-bottom to the
down-from-top expected by these puzzles, because it doesn't work right
on GNUstep. Instead, we run the API in its default mode (probably a
more robust choice anyway) and translate coordinates manually in the
front end.

In the process, I've separated the processButton: method into one for
mouse buttons and one for keys, since that was the easiest way to
ensure the coordinates passed to the mid-end for the latter are -1,-1
_after_ translation rather than before.

[originally from svn r9502]