Fix #1205.

Fix #1205.

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

* input/regression/metronome-range.ly, metronome-text.ly:

  replace explicit tempo changes using context props with exported events

* lily/metronome-engraver.cc:

  listen to TempoChangeEvent; set as cause for MetronomeMark; pass to metronomeMarkFormatter
  proc instead of explicit text, duration and count

* lily/parser.yy (tempo_event):

  simplify rule; use single constructor for all types

* scm/define-context-properties.scm (all-user-translation-properties):

  emend metronomeMarkFormatter description
  remove tempoUnitCount, tempoUnitDuration and tempoText definitions

* scm/define-event-classes.scm (event-classes):

  add tempo-change-event

* scm/define-music-display-methods.scm:

  rework display method for \tempo

* scm/define-music-properties.scm (all-music-properties):

  change type predicate for 'metronome-count

* scm/define-music-types.scm (music-descriptions):

  add TempoChangeEvent definition

* scm/ly-syntax-constructors.scm:

  rework `tempo' constructor; use for all \tempo styles
  remove `tempoText'

* scm/song.scm (tempo->beats):

  extract tempo from TempoChangeEvent

* scm/translation-functions.scm (format-metronome-markup):

  read tempo properties from event instead of passing as separate args