improved cue support

[Werner LEMBERG]

Consider the following situation:

  Main Voice:  c1        |

  Cue Voice 1: g4 r2.    |

  Cue Voice 2: r2 e4 g e |

I want to have this appear in the part:

   g4 e g e |
   c1       |

with other words, the first two quarters are taken from cue voice 1,
and the last two quarters from voice 2.

It's straightforward to insert two \cueDuring commands to get the
proper cues from cue voice 1 and 2.  The very problem is the main
voice since the whole note has to be split into two half notes, the
first one has to get the appearance of a whole note, and the second
one must be invisible.

Any idea how this could be avoided?  My idea is to have a list which
tells lilypond the time spans and the associated cue voices.  As an
additional benefit, this would allow to have more than a single cue at
the same time.  Here my proposed syntax -- of course, I have no idea
whether it is possible to implement that.

  \cue #'((<name1> <updown1> <skip1> <active1>)
          (<name2> <updwon2> <skip2> <active2>)
          ...)
       { <main voice> }

`skip' and `active' are simply arguments to the `make-moment'
function, but maybe you have a better idea.

Example:

  \cue #'((CueOne UP nil   (1 4))
          (CueTwo UP (1 4) (3 4)))
         { c1 }


    Werner