Use correct velocities on dynamics imported from MuseScore version 2 files

See discussion at https://musescore.org/en/node/290470.

This is basically how a dynamic with text before is entered into version 2

        <Dynamic>
          <subtype>other-dynamics</subtype>
          <text>poco <sym>dynamicForte</sym></text>
          </Dynamic>

For import from version 2:
If a velocity is explicitly set, it should be used as is currently the case.
Since, for an edited dynamic, the subtype is "other-dynamic" a test should be run to check for dynamic symbols, in this case "dynamicForte". Once a second one is found, then I agree the velocity, if not explicitly set, should be set to zero, but if only one dynamic is found then the velocity should be set to its default rather than zero.

I consider this a major issue because there are a lot of "subito p" type dynamics out there and their velocities are all being set to 0 on import.