Display-based word wrapping

[Chong Yidong]

I've checked in Kim F. Storm's display-based word-wrapping patch, with
some modifications.  The word-wrapping is controlled by the `word-wrap'
per-buffer variable.  It is a form of continuation line, so e.g. will
not be performed if lines are truncated rather than continued.

The default is nil.  More work will have to be done to make this really
useful for editing documents with long lines: (i) the minor mode for
screen-based editing commands being discussed in another thread, (ii) a
way to change the continuation/wrapping/truncation margin, and (iii) a
way to customize the fringe indicators to avoid displaying tons of
continuation line indicators.

I have also changed truncate-partial-width-windows to accept integer
values specifying a window width below which to truncate lines, and
changed the default to 30.

I believe the patch should not affect the default behavior where
word-wrap is nil, but please be on the lookout.  Problem reports are
welcome.