Heres a new version of the patch. New stuff:

- The interface is now an alist tied to the window.
I think Stefan prefered this.

acessors:
set-window-parameter
window-parameter

- set "pin" to t, and the window will not go away on delete-other-winows
- set "group" to something, and all windows with the same value will be
considered in the same window group, which affects other-window for
instance.

- I also have some hackish elisp code to show how the interface works.

This is all only lightly tested, but looks IMHO promising.

Issues:

- I didn't adress Klaus concern with switch-to-buffer yet, I'm not sure how
to proceed there.

- I broke an optimization, marked as FIXME in windows.c. It doesnt look
  especially important so I wont not spend time on it until everything
  works properly.