Makes line-breaking possibilities more available to grobs (issue 5353047)

[mtsolo]

Reviewers: ,

Message:
Hey all,

This is the first step in a series of patches that I'm gonna try to
develop to create a TextScriptSpanner - a TextScript that automatically
word wraps to the end of a line break and thus doesn't cause oddly
deformed lines when markups fall towards the end of a line.

To do this, the pure height function will need to know the estimated
distance between X columns, which only works if the pure height function
can access the results of the Simple_spacer.  This reorganizes the code
so that any grob, in theory, can access these results.

Cheers,
MS

Description:
Makes line-breaking possibilities more available to grobs

Please review this at http://codereview.appspot.com/5353047/

Affected files:
  A lily/column-description.cc
  M lily/constrained-breaking.cc
  A lily/include/column-description.hh
  M lily/include/item.hh
  M lily/include/paper-column.hh
  M lily/include/simple-spacer.hh
  M lily/include/system.hh
  M lily/item.cc
  M lily/paper-column.cc
  M lily/simple-spacer.cc
  M lily/system.cc