sourceView: collapsing staffs in score

[riedde]

We talked a couple of times how to deal with scores that are quite big if only a specific Part is of interest. Would it be an option to set <staff> into <measure> linking it to a <zone> which marks the "zone of interest"? Technically this could be solved using the partsView for scores containing staff/@facs (as an option). This would allow filtering staffs.