This patch implements "something like" Mac OS X's application cycling mode. It lets you hit a separate key to switch to the next window of the same class as the currently-focused window.
Initial version of this patch fixed by Samir Saada.
For those not familiar with the way Macs cycle windows, the Command-Tab sequence (Alt-Tab elsewhere) switches between DIFFERENT application windows and Command-Grave (key above tab) switches between windows owned by the SAME application as is currently focused. So if you had three Safari and two Finder windows open, and Safari had focus, Command-Tab would switch to Finder; Command-Tab would switch back to Safari; Command-Grave would switch to a different Safari window etc.
This patch implements "something like" the above by only populating the switchpanel with windows matching the currently-focused WWindow's wm_instance and wm_class when the new cycling mode is activated. In practice this means you can switch to The Next XTerm or The Next Mozilla Window using this method.
The configuration names for these new shortcuts are GroupNext and GroupPrev. The patch tells WPrefs.app about them. Of course switching to The Next Window is still possible with the (unchanged) FocusNext and FocusPrev keys.
Opinion: I find the Mac way of doing things annoying (because cycling to The Last Window requires you to remember which key to use depending on what type of application has focus) but at the same time very useful (because easily switching to The Next Terminal.app is nice). With this patch you can switch to The Next Window or The Next XTerm - the best of both worlds!