Rewrites last-grob-cause function (issue 7092058)

[dak]

https://codereview.appspot.com/7092058/diff/1/scm/output-lib.scm
File scm/output-lib.scm (right):

https://codereview.appspot.com/7092058/diff/1/scm/output-lib.scm#newcode57
scm/output-lib.scm:57: (define-public (last-grob-cause grob)
The name is quite clearer, and the semantics make sense.  It is
conceivable that it would be more useful to return #f instead of the
grob itself when there is no preceding grob cause.  But I am not really
sure about that, and this variant _is_ consistent as well.  However, I
should be surprised if previous callers won't need to be notified of the
renaming as well.  This has been committed so recently that, in spite of
this being a public function, a convert-ly rule seems like overkill to
me (it would basically cover one version or less).  But the callers
introduced with the same commit will need renaming, anyway.

https://codereview.appspot.com/7092058/