Fix 1198: Fix displayLilyMusic for \time and #(set-time-signature)

Fix 1198: Fix displayLilyMusic for \time and #(set-time-signature)

create TimeSignatureMusic to allow the display to work properly

* scm/define-music-callbacks.scm
  Create a new file for music callbacks that are used by iterators to
  create music events.  Moved multimeasure rest and ottava callbacks from
  scm/define-music-types.scm.
  Added callback for TimeSignatureMusic.

* scm/define-music-display-methods.scm
  define a display method for TimeSignatureMusic
  remove extra-display-methods for ContextSpeccedMusic and Timing that
  caught \time

* scm/define-music-properties.scm
  define beat-structure as a music property

* scm/define-music-types.scm
  Move callbacks for multimeasure rests and ottava to scm/define-music-callbacks.scm
  add TimeSignatureMusic

* scm/lily.scm
  Add scm/define-music-callbacks.scm to load list

* scm/music-functions.scm
  replace calls to make-beam-rule-time-signature-set with calls
  to make-music 'TimeSignatureMusic