Enable widescreen window layout

[ianfixes]

From the original project's BasiliskII/src/MacOSX/ToDo.html:

Widescreen window layout (suggestion by Michael Franz), so that users with widescreen displays can squeeze a bigger emulated screen in. I have a mock up of this that can be pasted in MainMenu.nib. Ideally, this would be via a generalised "theme" facility, but who has time for that :-)

No further information is available; it's also possible that this issue has been addressed in the time since that 2007-07-14 revision.