preexisting MEI: keep processing instructions intact

[bwbohl]

When loading a preexisting MEI file into Vertaktoid it strips existing processing instructions from the file, this should not happen.

• vertaktoid deletes existing xml-model processing instruction in XML-frontmatter, e.g.

  <?xml version="1.0" encoding="UTF-8"?>
  <?xml-model href="https://music-encoding.org/schema/4.0.1/mei-CMN.rng" type="application/xml" schematypens="http://relaxng.org/ns/structure/1.0"?>
  <?xml-model href="https://music-encoding.org/schema/4.0.1/mei-CMN.rng" type="application/xml" schematypens="http://purl.oclc.org/dsdl/schematron"?>

  is being reduced to

  <?xml version="1.0" encoding="UTF-8"?>

  even when only looking at the file and not modifying anything in vertaktoid.

[bwbohl]

• processing -instructions, e.g. <?TODO ?>, are generally being removed from the file when opening it with vertaktoid