For hairpins without dynamics, change velocity one dynamic level
As requested at https://musescore.org/en/node/87261.
Current behavior: a hairpin without a specified velocity change will have no effect on playback unless there are dynamic markings before and after it.
Requested behavior (incidentally Sibelius behavior): if there is no dynamic marking after a hairpin, volume should change by one dynamic level "(e.g. a crescendo hairpin that starts at a
prevailing mp will go to mf, while a diminuendo hairpin that starts at a prevailing ff will go to f)."