Prall and trill in "articulate" script

[Gilles]

Hello.

It's the first time I use the "\articulate" command.
Very nice improvement on the plain MIDI rendering!

Two remarks:
1. "prall" is interpreted in a way that (IMO) better corresponds
   to "prallprall" (which itself is not implemented).
   [A comment in the code indicates that the "inverted mordent"
   option was not retained.]
2. A "trill" on a tied note only lasts as long as the first written
   note.  The problem is alleviated by using a trill span, but then
   the trill is stopped at the start of the last written note's
   duration.


Thanks,
Gilles