MusicXML: Convert colors in text markup using \with-color #(rgb-color ...)

MusicXML: Convert colors in text markup using \with-color #(rgb-color ...)

Now that we have the rgb-color scheme function in git, make use of it in
text markup conversion. In MusicXML, the color is given as #AARRGGBB
or #RRGGBB, we simply ignore the alpha channel, extract the r, g and b
values and scale them to the 0.0 - 1.0 range