Engravers deciding about grob classes

[Jean Abou Samra]

Hello,

In https://gitlab.com/lilypond/lilypond/-/merge_requests/818,
I am proposing a change that will affect Scheme engravers.
Basically, with that patch, the same grob type can be used
to make grobs of any class. For example, FootnoteItem and
FootnoteSpanner are unified into Footnote. It is the job
of engravers to choose whether to make an Item or a Spanner.
ly:engraver-make-grob is thus replaced with ly:engraver-make-item
and ly:engraver-make-spanner.

Just a heads-up for Scheme power users so you have a chance
to comment.

Regards,
Jean