's' keyboard shortcut should toggle a slur off if it's spanning exactly the selected range
Steps to reproduce the issue:
1) Write some notes
2) Select a range, press 's' (→ that range is slurred)
3) Press 's' again
* expected behavior: the slur disappears (consistent, for example, with the alt+s shortcut, which toggles on/off the staccatto articulation)
* actual behavior: another slur is created on top of the new one.