[MusicXML] split words and symbols

[rettinghaus]

This fixes export of text mixed symbols to corretcly encode symbols as symbol.

[miiizen]

This looks good, we should use it for all words within a <direction-type> though

[rettinghaus]

I'm a bit hesitent, because text formatting doesn't work here:

With this solution the first case gives the expected result:

        <direction-type>
          <words>To</words>
          <symbol>coda</symbol>
          </direction-type>

But when you change the font the result keeps the xml formatting:

        <direction-type>
          <words>&lt;font size=&quot;12&quot;/&gt;&lt;font face=&quot;Didot&quot;/&gt;To</words>
          <symbol>coda</symbol>
          </direction-type>

The existing function returns this:

        <direction-type>
          <words font-family="FreeSerif">Standard </words>
          <words font-family="Leland Text"></words>
          </direction-type>

Ideally we would combine the two approaches, but for now I wouldn't touch the existing implementation.

[Jojo-Schmitz]

Backport to 4.7 in #32685