Movable and selectable views in scrolling windows

[Peter Szolovits <psz@lcs.mit.edu>]

Due to apparent widespread interest in this code, I have posted it to
cambridge.apple.com as /pub/MCL2/contrib/movable-view.lisp.  The code
defines two CLOS mixin classes, movable-view and selectable-view,
which support dragging and selection.  The two work together in that
if several objects are selected, they are all dragged together.  It
also works both in unscrollable windows, wherein the frame constrains
movement, and in scrollable windows, where (what I think is the
Apple-ordained style of) dynamic scrolling is implemented.

It's still prerry rough, and doesn't do all the things I'd like, but
it seems to work.  Comments, suggestions, improvements and
replacements welcome.

Peter Szolovits
MIT Lab for Computer Science