Heres a patch to implement Window-groups in Emacs, which I submit for
review.  Theres comments in the patch to explain what Window-groups are
in practice. In brief, the aim is to create Emacs primitives useful to ECB
like frameworks. 

The code builds on top of the already comitted window-properties code.
The patch has C level changes and an elisp interface.

To understand the design rationale, please read previous rather
long-winded window group threads. The patch is a joint effort between
me, Stefan Monnier, Chong Yidong, and Martin Rudalics.