Fix bracket drag'n'drop

Hiding a bracket spanning a single staff, hides also a bracket just dragged and dropped from the palette.

Fixed by distinguishing during System::layout2() between a multi-staf bracket accidentally covering a single visible staff and a bracket really spanning a single staff.

this also allows to have a bracket intentionally spanning a single staff, should the user need it.
Merge pull request #608 from mgavioli/Fix_bracket_dragndrop

Fix bracket drag'n'drop