In the previous generated player bindings the position-updated was Send+Sync but
only Send is actually needed. In addition to this change the position-updated,
duration-changed and seek-done signals now invoke their handler with a
gst::ClockTime value instead of u64.
And also implement a FormatValue type that holds a value together with
its format to make usage of the positions/durations/seek positions/etc
more convenient and safe.
Fixes https://github.com/sdroege/gstreamer-rs/issues/3
