Fix #35136 : Tremolo bar always on top staff + crash on delete

Fix #35136 : Tremolo bar always on top staff + crash on delete

A) A tremolo bar is always displayed on the top staff, regardless of the staff is it applied to:
corrected by adding the ON_STAFF flag to a new `TremoloBar` object

B) Deleting a tremolo bar crashes the programme:
corrected by adding the relevant case to the `Segment::remove()` method