attachment points for vertical spacing dimensions

[Mark Polesky]

WRT the flexible vertical spacing dimensions, the upper
attachment points for 'space and 'minimum-distance currently
align with the Y-coordinate of the origin (0,0) of the upper
item.  For systems this is the middle line of the nearest
staff, and for markups this is the highest point of the
markup.  In the newest docs (NR 4.1.2 as of yesterday),
these are called "reference points".

I think that most of the resulting dimensions are what the
user would naturally expect them to be, except when the
upper item is a title/markup.  In these cases, I think the
most natural attachment point would be the *bottom* of the
upper markup.

This applies to 3 of the 8 flexible vertical dimensions:
  * after-title-spacing
  * between-title-spacing
  * bottom-system-spacing

The proposed change to after-title-spacing needs no comment.

For between-title-spacing however, I should mention that if
the upper attachment point (of 'space and 'minimum-distance)
is moved to the bottom of the upper markup, then the
'padding value is basically rendered redundant.  In that
case, 'padding would only influence the spacing if it were
larger than 'minimum-distance, and making 'padding larger
than 'minimum-distance is generally pointless since that in
turn would render 'minimum-distance redundant.  That being
said, I don't think this is a problem; the spacing behavior
would still be more natural IMO.  And a simple explanation
for this unique case could be added to the docs.

Of the three, bottom-system-spacing is slightly more
complicated, since it currently controls the spacing below
systems *and* markups, when either is the last on a page.
So the natural attachment point for systems would remain the
same, but would be shifted to the lowest Y-coordinate for
markups (ideally).

Personally, I think we should add a new variable to control
the spacing between a markup and the bottom margin.  We
could call it bottom-markup-spacing for now, but see this
post for my proposed variable renaming:

http://lists.gnu.org/archive/html/lilypond-devel/2010-10/msg00070.html

Let me know what you guys think; it would be nice to achieve
consensus on this one.

Thanks.
- Mark