Rewrite the vertical layout of staves/systems.

Rewrite the vertical layout of staves/systems.
This combines the positioning of systems and staves into
a single pass of a rod-and-spring problem. As a consequence,
the spacing of staves within a system is much more configurable.

Most properties related to vertical spacing have been removed
(eg. page-top-space, between-system-space, between-system-padding)
and replaced with more powerful versions (eg. first-system-spacing,
first-system-title-spacing, between-system-spacing).

FIXME: annotate-spacing is broken

[Patrick McCarty, 2009-07-27 05:42:20]

I don't really understand the code from this patchset, but I just have one quick
comment.

Thanks,
Patrick

http://codereview.appspot.com/97119/diff/1/22
File lily/staff-grouper-engraver.cc (right):

http://codereview.appspot.com/97119/diff/1/22#newcode21
Line 21: {
Are engravers allowed to inherit code from other classes?

I'm asking because there is a comment in slur-engraver.cc:

   (on principle, engravers don't use inheritance for code sharing)

If the inheritance is okay, then the comment (and others, if any) should be
removed from the slur-engraver.cc.