Let's make TextSpanners easier to use!

[Valentin Villenave]

Greetings everybody,
as Graham recently noted, TextSpanners are currently painful to use:

{
  \override TextSpanner #'bound-details #'left #'text = "rit."
  b1\startTextSpan
  e'\stopTextSpan
}

We really should provide our users with a shortcut: a single command,
less powerful, less flexible but simpler to use. Ideally , this
command would take the spanner's text as an argument, and start the
spanner automatically. (The \override way would still exist for
power-users.)

I've spent some time trying to write such a command, but I'm stuck.

%%%%%%%%%%%%%%%

textSpan =
#(define-music-function (location parser  txt) (string?)
(let* ((m (make-music 'TextSpanEvent
             'span-direction -1)))
   (ly:music-set-property! m 'tweaks
    (acons 'bound-details '((left . ((text . "txt"))))
     (ly:music-property m 'tweaks)))
 m))

\relative{
b'1\textSpan #"rit"
e, f\stopTextSpan
}

%%%%%%%%%%%%

The annoyances with my code are:
  - it replaces the whole bound-details alist, and therefore all
alignment-related settings are lost
  - for some reason I don't understand, the txt variable can't be
passed as a text string to the 'text property (hence the double quotes
in my code, otherwise it doesn't work).
  - it doesn't take a markup as argument.

If any of you guys has some ideas to fix these issues (or improve my
concepts), it would be great if we could come up with a command to
include in future versions of LilyPond!

Cheers,
Valentin