[XY]-core-extent and general_alignment (issue 2613) (issue 6308093)

[janek . lilypond]

Reviewers: MikeSol, joeneeman,

Message:
A pdf showing what this patch does is attached to tracker issue:
http://code.google.com/p/lilypond/issues/detail?id=2613#c2

Please review!

Description:
[XY]-core-extent and general_alignment (issue 2613)

This adds a versatile method for specifying alignment of grobs.

Please review this at http://codereview.appspot.com/6308093/

Affected files:
  M lily/dimension-cache.cc
  M lily/grob.cc
  M lily/include/dimension-cache.hh
  M lily/include/grob.hh
  M lily/include/self-alignment-interface.hh
  M lily/self-alignment-interface.cc
  M scm/define-grob-properties.scm
  M scm/define-grobs.scm