Notation Type for GP7 style tabulature info (string/fret)

[oaken-source]

GuitarPro7 has added a MusicXML import/export feature, and they have added their own method of specifying tabulature info in the XML within the <notations> tag, like this:

        <technical>
   <?GP7 <root><string>5</string><fret>5</fret></root>?>
        </technical>

I have added a GP7Tabulature Notation type to inject this type of notation into the generated XML tree. Would a feature like this be appreciated? If so, I will draft a pull request with all the details.