New Feature: Include text in \tempo indications

New Feature: Include text in \tempo indications

A tempo indication in general is either a text markup, a note=count or both.
So far, lilypond only supported the note=count type of tempo indications in
its \tempo command.

This patch extends the \tempo function to include a text string, too. It
allows any of the following types of tempo settings:
    \tempo 4=120
    \tempo "Allegro" 4=120
    \tempo "Allegro"
    \tempo \markup{\italic \medium "Allegro"}
etc.

What it does:
  - Extend the parser to allow the above forms for \tempo
  - Add a tempoText property, similar to tempoUnitCount
  - Metronome_mark_engraver uses this property and checks whether it has
     changed
  - Extend the metronomeMarkFormatter to take four arguments (text, duration,
    count, context) and print either the text, the note=count or text
    (note=count), depending on whether the properties are set to sensible
    values.
  - Add a tempoHideNote property to hide note=count in the metronome mark
  - Extend the define-extra-display-method to also check for tempoText and
    produce any of the allowed \tempo forms when you use \displayLilyMusic
  - Added a convert-ly rule to warn the user about the change to the signature
    of the metronomeMarkFormater function