Re: Caches the interior skylines of vertical axis groups and systems. (issue 7185044)

[mtsolo]

People can review this if they have time...it is a lot of code, but it
is more a progress report than anything else.

To resume a bit of what's been said before: once translate_axis is
called, the dim_cache of grobs is set and it is very difficult to work
with approximations.  This makes cross-staff grobs very difficult to
deal with in vertical placement.  The goal is to eliminate this property
call for outside-staff placement so that pure skylines can be used in a
first pass without setting the Y-offset.  Then, in the second pass, real
skylines will be used and the Y-offset will be set.

https://codereview.appspot.com/7185044/