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
Have to hide some properties again at a later time, which have actual
getter/setter functions already. We only need the notification signals
for those.
Depends on the GIR patch in https://github.com/gtk-rs/gir/pull/441
