Mark_performer

Enhancement 1: Mark_performer

Create MIDI Marker events whenever the Mark_tracking_translator chooses a mark. Markup is formatted as if for printing and then lossily converted to a string for MIDI.

markup->string handles segno, coda, and varcoda signs, but it produces the same output for coda and varcoda. I decided against decorating the string when the mark formatter uses \box or \circle, but I recorded the idea in TODO comments.

Closes #6160 (closed).

Enhancement 2: Lyric_performer: accept markup

Define a constructor for Audio_text that accepts a SCM and runs markup->string on it. Use it in the Lyric_performer. (This is the same transformation used for marker text.)

Maintainability changes

• Add Performer::announce<T> () to hide new. Call it in some obvious places.
• Audio_element_info: C++11 member initialization
• Label many Audio_item subclasses as final