* Add new procedure `window-override-message' that returns the

* Add new procedure `window-override-message' that returns the
  override message, or #F if none.  This is used to implement
  `current-message', which operates on the current typein window.
  `clear-message!' renamed to `clear-current-message!'.
  `set-message!', renamed to `set-current-message!', now accepts #F as
  an argument, in which case it acts like `clear-current-message!'.

* Split `select-buffer-in-new-screen' into two parts: a procedure
  `make-screen' that generates a new screen, and a call to
  `select-screen'.  The new procedure `make-screen' replaces the
  procedure of the same name which is now considered internal to the
  screen abstraction.

* Change `select-screen' not to abort to top level, to run the
  `select-buffer-hook', and to transfer the typein override message
  from the previously-selected screen to the newly-selected one.  The
  X screen event handler now does the abort to top level, since it is
  still needed in that case.

* Eliminate the typein bufferset -- typein buffers are shared by all
  screens.  Display of the typein buffers is suppressed in
  non-selected screens by a blank override message.  This has the
  drawback that direct update of the typein window does not work if
  there are multiple screens, which makes typein feel sluggish.

* Implement procedures `screen0', `screen1+', `screen-1+', and
  `screen+' for moving around the screen list.

* `buffer-list' no longer copies its result -- don't clobber it!

* New procedure `change-selected-buffer' makes the handling of buffer
  selection more uniform.

* Eliminate cacheing of `editor-input-port' from "input.scm".  Just
  extract the input port from `current-editor' every time.  Change the
  keyboard reader to use `input-port/read-char' instead of
  `read-char', since the former is faster.

* Redesign the `button' abstraction to make it cleaner and more
  general.  Rename the `buttonN-down' and `buttonN-up' variables to
  `x-buttonN-down' and `x-buttonN-up'.  Change button-handling code so
  that up buttons don't beep if they aren't bound to commands.

* Updating of an X screen is now terminated if a non-keypress event
  arrives while checking for update preemption.  This is done by
  throwing out of the update.  I believe this test only happens in
  places where it is safe to do this.

* Make screen highlight control independent of the screen type.
  Change screen abstractions so that screens support two operations,
  one to turn on highlighting, and the other to turn it off.