Increase the maximum number of lines in a staff

See discussion in https://musescore.org/en/node/140446

It might indeed be possible to this even for MuseScore 2.0.4 branch, I tryed it out with a modified build from master branch and 2.0.3 was able to read the file and show the additional staff lines.

I'm unsure whether or not https://github.com/musescore/MuseScore/blob/d85ff2851b8f6a075c6d7b31549… comes into play here too and whether there are other places to change or reasons not to do it at all, like e.g. MusicXML export?