vertical spacing: Rename dimensions. (issue2505041)

[markpolesky]

Reviewers: ,

Message:
Sorry this took so long.  I uploaded all 6 commits
as separate patch sets so it's easier to review.
Patch set 7 combines all the changes into one set.

Okay to push?
- Mark

Description:
vertical spacing: Rename dimensions.

Please review this at http://codereview.appspot.com/2505041/

Affected files:
  M Documentation/de/notation/spacing.itely
  M Documentation/es/notation/spacing.itely
  M Documentation/fr/notation/spacing.itely
  M Documentation/notation/spacing.itely
  A input/regression/page-breaking-markup-padding.ly
  A input/regression/page-breaking-markup-padding2.ly
  A input/regression/page-breaking-markup-padding3.ly
  M input/regression/page-breaking-min-distance.ly
  D input/regression/page-breaking-title-padding.ly
  D input/regression/page-breaking-title-padding2.ly
  D input/regression/page-breaking-title-padding3.ly
  M input/regression/page-spacing-loose-lines-between.ly
  M input/regression/page-spacing-loose-lines-header-padding.ly
  A input/regression/page-spacing-top-markup-spacing.ly
  M input/regression/page-spacing-top-system-spacing.ly
  D input/regression/page-spacing-top-title-spacing.ly
  M input/regression/paper-nested-override.ly
  M input/regression/stem-length-estimation.ly
  M input/regression/system-overstrike.ly
  M lily/constrained-breaking.cc
  M lily/include/constrained-breaking.hh
  M lily/page-breaking.cc
  M lily/page-layout-problem.cc
  M ly/paper-defaults-init.ly
  M python/convertrules.py
  M scm/page.scm
  M scm/paper-system.scm