552df47 Add `define-event-class' and scheme spanner regtest

[David Kastrup]

The commit mentioned in the subject line is just not right.  The purpose
of a definition like "define-event-class" is not, as far as I am
concerned, to provide an attack vector where one can, given suitable
ingenuity, hack together code that fiddles with internals to the effect
of defining an event class, bypassing all the code intended for that
purpose in Lilypond.

The purpose should be to make it feasible to define an event class.  If
the regtest has to hack together its own `add-grob-definition' function
doing extensive surgery on internals, that goal is not met.

The regtest should be accessible to a lower breed of humans than the
programmers of the core functionality.  In order to avoid a maintenance
nightmare, it should preferably be using the same interfaces as the
Lilypond core.

Now the Lilypond core currently uses the interface "make the programmer
create a giant list in one central place that is already sorted and
prefilled with hand-crafted entries".

Tell you what?  I think that most (likely all) of these scm/define-*.scm
should be obliterated.  Instead, the relative entries should be
generated by appropriate user-accessible commands like
`define-event-class' near the place that the event classes are actually
used.  They can record the defining location for the purpose of cross
referencing generation.

Ok, enough ramblings.  While not all of this needs to be implemented
right away, the current definition of `define-event-class' is clearly
insufficient if a not completely trivial usage example (what the regtest
is) requires lots of additional internal surgery.

-- 
David Kastrup