Implement Grob::event_cause, Grob::ultimate_event_cause

Implement Grob::event_cause, Grob::ultimate_event_cause

Those were implemented in Grob_info only previously which does not make a lot of sense,
given that they don't access anything particular to Grob_info.


Also contains commit:

Use Grob::event_cause and Grob::ultimate_event_cause where useful



Frankly, this one has been bugging me on and off for years.  I just
had one compilation error too much today.

[hahnjo, 2020-02-20 15:24:31]

LGTM, nice cleanup

[hanwenn, 2020-02-20 23:13:54]

LGTM

[Dan Eble, 2020-02-21 00:13:25]

LGTM

https://codereview.appspot.com/559500043/diff/559510043/lily/grob.cc
File lily/grob.cc (right):

https://codereview.appspot.com/559500043/diff/559510043/lily/grob.cc#newcode733
lily/grob.cc:733: while (unsmob<Grob> (cause))
I appreciate that issuing warnings is not performance-sensitive, and that you
simply transplanted this code, but what do you think about avoiding repeating
the unsmob, to set a good example?  Something like ...

    while (const Grob *g = unsmob<Grob> (cause))
      {
        cause = g->get_property ("cause");
      }

[dak, 2020-02-21 00:32:07]

https://codereview.appspot.com/559500043/diff/559510043/lily/grob.cc
File lily/grob.cc (right):

https://codereview.appspot.com/559500043/diff/559510043/lily/grob.cc#newcode733
lily/grob.cc:733: while (unsmob<Grob> (cause))
On 2020/02/21 00:13:25, Dan Eble wrote:
> I appreciate that issuing warnings is not performance-sensitive, and that you
> simply transplanted this code, but what do you think about avoiding repeating
> the unsmob, to set a good example?  Something like ...
> 
>     while (const Grob *g = unsmob<Grob> (cause))
>       {
>         cause = g->get_property ("cause");
>       }

Not going to use const here since it is pointless while we have nothing like
const_unsmob, and would not use that order anyway since I consider it misleading
(it suggests a declaration of a constant, rather than a pointer to a constant). 
Other than that, fine.

[dak, 2020-02-21 00:40:23]

Save one unsmob.

[dak, 2020-02-21 00:54:47]

Save one iteration!  SCNR