Trails should be able to shrink over time

[JeremyLeland]

Trail should gradually shorten if ship slows down, or if bullet hits something (make trail a particle and let it finish shrinking, rather than immediately vanish).

Make public goalLength Make maxLength private Add dLength (public?) If goalLength < maxLength then maxLength -= dLength * dt Need update to take dt as well as (optional?) parent (only update head if parent not null)

[JeremyLeland]

This was done in recent commit