Musicxml import: support <backup> and <chord>, support slurs, ties and multisyllabic words

[oxygen-dioxide]

• Support for <backup> and <chord> elements in MusicXML.

  • <backup> moves the current time position backward by a specified duration.
  • <chord> sets the start time of the current note to the beginning of the previous note, forming a chord instead of starting after the previous note ends.

• Support for ties and slurs in MusicXML.

  • Ties will not create new notes; instead, the duration is added to the previous note with the same pitch.
  • Slurs are parsed as +~ to extend the current syllable.

• Support for multi-syllable lyrics.

  • The full word is written to the first note of the syllable, and subsequent notes use + to assign the following syllables.