Expose Score's utick2utime and utime2utick to plugins

There are several interfaces (in particular, Segment) currently exposed to plugins that give a tick, but only Cursor has the ability to give plugins the current time. It would be great if plugins could easily convert between ticks and time.

I think the alternative is for the plugin to try to find all the tempo and time signature objects and use those to calculate time, and I'd prefer not to do that.