MusicXML: Implement staff-lines setting (i.e. # of lines of a staff)

MusicXML: Implement staff-lines setting (i.e. # of lines of a staff)

-) Add possible context modifications to staves
-) convert a global staff-lines attribute to a contex mod inside \with
-) Convert a staff-lines attribute midway through the part as
    \stopStaff \override ... \startStaff
-) Add regtest for all these cases