Fix #1205. (issue2145047)

[n . puttock]

Reviewers: ,

Message:
Hi,

This patch changes the internal representation of \tempo so it uses an
event to trigger creation of a metronome mark.

Please review.

Thanks,
Neil

Description:
Fix #1205.

Reinstate an event for \tempo, instead of relying on context property
changes.

Please review this at http://codereview.appspot.com/2145047/

Affected files:
  M input/regression/display-lily-tests.ly
  M lily/metronome-engraver.cc
  M lily/parser.yy
  M scm/define-context-properties.scm
  M scm/define-event-classes.scm
  M scm/define-music-display-methods.scm
  M scm/define-music-types.scm
  M scm/ly-syntax-constructors.scm
  M scm/song.scm
  M scm/translation-functions.scm