group checklist toggle (post-summer-break feature request)

[Scott Otterson <scotto@u.washington.edu>]

At first, I wasn't sure I'd use the new '[X]' checklists, but now I've 
got them sprinkled all over my .org files -- they're especially handy 
for multi-step tasks which are done repeatedly.  

When I repeat a multi-step task I've done before, one of the first 
things I do is to uncheck all of the [X]'s.   So, I'd like C-c C-c to 
toggle all of the [X]'s within a selected region, similar to the way you 
can now indent or unindent selected region headlines.  Toggling all of 
the checks under a headline might also be useful, but maybe less so.

Thanks for all the great work,

Scott