Asserts that footnotes are being numbered correctly. (issue 5433065)

[mtsolo]

Reviewers: ,

Message:
Hey all,

This patch helps make sure that the footnote auto-number regtests
actually test what they purport to be testing by allowing a callback to
check if footnote grobs are receiving the correct automatic number.

Cheers,
MS

Description:
Asserts that footnotes are being numbered correctly.

Please review this at http://codereview.appspot.com/5433065/

Affected files:
  A input/regression/footnote-auto-numbering-vertical-order.ly
  M input/regression/footnote-auto-numbering.ly
  M lily/include/page-layout-problem.hh
  M lily/page-layout-problem.cc
  M scm/define-grob-interfaces.scm
  M scm/define-grob-properties.scm