Slurs for multi-staff selections

1. Open back-bc2.mscx demo file
2. Lasso-select a group of notes in several staves
3. Press S to add slurs

Expected behavior: If a selection spans multiple staves (and contains more than two notes in the selection) then each staff should get its own slur.

Actual behavior: A single slur slur is added to the first staff. Sometimes the slur starts in the top staff and cuts across to a lower staff if a lower staff has the last note in the selection.