Provide \markupMap music function

Provide \markupMap music function

This applies a markup function to all markup music properties matching a
given music property path.

For example,

\new Voice { g'2 c'' }
\addlyrics {
  \markupMap LyricEvent.text
             \markup \with-color #red \etc
             { Oh yes! }
}

Export markup-function? and markup-list-function?


Add check-music-path function