Add "Cycle Through Windows" command

[inexorabletash]

Apple-` perhaps.

Suggested by Jay Edwards.

[inexorabletash]

Wrinkle: under Virtual ][, OpenApple-` cycles windows and Option-` is a "dead key" (as a way to insert accented characters like è).

Also, one of the select-and-activate-window routines paints the window twice. Otherwise... it's easy.