Use fixed-width digits

[ryandesign]

Clocker is evidently using standard proportional-width digits such that the width varies depending on the time. This is especially apparent if you show the seconds.

If my menubar is especially full of regular menus and menu extras, I've seen it happen that a menu extra will appear and disappear from second to second depending on the time as more or less space becomes available. This is distracting.

Apple's clock doesn't have this problem.

Clocker should ask for fixed-width digits. (monospacedDigitSystemFontOfSize:weight:?)