There's now get_result() instead of get(), and separate getters for only
getting the constructor arguments of each query (otherwise query
handlers will get useless values when trying to answer a query).
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