Handling of subdivisions with shortened beams

[Urs Liska]

One more from this round of review ...

I had changed the subdivision behaviour (which is already in 2.19.35)
for shortened beams.

c32 [ c c c c c c] r

(i.e. a beam shortened by 1/32) subdivided by 1/8 currently has *two*
beams at the subdivision - to indicate that the remaining group is
shorter than 1/8.

However, now I realize that we are not actually counting the *length* of
a group but rather its metric situation. And I think that's the right
way because this is how the performer gets the right information to
"navigate" complex situations.

OTOH this feature seems to have been welcomed during review, so I
wouldn't want to silently revert that now. I had the idea to also add an
option for this. If you consider the attached image, what would you suggest:

a)
Always do the first version (i.e. strictly beam according to metric
position)

b)
Always do the second version (i.e. shorten to indicate the shortened
length of the beam)

c)
Take a) as the default and provide a context property

?
Urs