Allow Scheme functions to be called inside of markup (issue 285070043 by address@hidden)

[thomasmorley65]

After some testings I'm quite happy with it, apart from one use case:

fooII =
#(define-scheme-function (arg)(markup-list?)
#{
  \markup \column $arg
#})

%% doesn't work:
\markup \line { "xy" \fooII { "does" "not" "work"} }
%% works
\markup \line { "xy" \fooII \markuplist { "whatever" "else" } }
\markup \line { "xy" \fooII #'("whatever" "else") }

I think I know why the first case doesn't work. Though, I have no good
idea how to explain it in the docs without telling a very technical
story, likely very confusing to the average-user.
Additionally, having to add \markuplist as in the second example is not
nice. Only a little better is the need to revert to scheme as done in
the third example.

What other problematic cases can you predict deductively?

https://codereview.appspot.com/285070043/