Make part-symbol repeatable

[rettinghaus]

Currently it is only possible to specify a single part-symbol in attributes. That makes it impossible to encode Rachmaninoff’s Prelude in C-sharp minor:

It should be allowed Zero or more times:

<attributes>
  <part-symbol top-staff="1" bottom-staff="2">brace</part-symbol>
  <part-symbol top-staff="3" bottom-staff="4">brace</part-symbol>
  <clef number="1">
    <sign>G</sign>
    <line>2</line>
  </clef>
  <clef number="2">
    <sign>G</sign>
    <line>2</line>
  </clef>
  <clef number="3">
    <sign>F</sign>
    <line>4</line>
  </clef>
  <clef number="4">
    <sign>F</sign>
    <line>4</line>
  </clef>
</attributes>

Compare #63.