Hairpins: allow user to set start and end note velocities

Presently, for hairpins, the user can set the velocity change; but it would also be useful in certain cases to have an alternative method (in the Inspector) of being able to set the Start velocity and End velocity instead. For example, when there is a series of crescendo (or diminuendo) hairpins, with no intervening dynamic markings, this would allow the user to reset the start velocity of each hairpin.