Replace AppSrcCallbacks/AppSinkCallbacks with a builder so that no empty closures have to be provided for unused callbacks

Philippe Normand authored Dec 10, 2017 and Sebastian Dröge committed Dec 10, 2017

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.

Otherwise we conflict with GLib translation traits, which causes some
annoyances.

FormatValue is now renamed to GenericFormattedValue and the API slightly
changed. In addition there is now a FormattedValue trait, and a
SpecificFormattedValue trait plus types for Bytes, Buffers and the
existing ClockTime.

This allows to create functions like
  Pad::query_duration<F: SpecificFormattedValue>() -> Option<F>
and doesn't require the caller to unwrap the generic value anymore,
which is completely unneeded in these cases.

In addition, Segment became FormattedSegment<T> with API to
upcast/downcast between the specific formatted values and the generic
formatted value. This greatly simplifies usage of Segments.

This is guaranteed to be of the correct type.

We need to scale the number of samples, not the buffer size.

And implement the Value traits for ClockTime

Get rid of unneeded transmutes and actually initialize flags/enum
GValues with the correct type instead of using integers.

Let Sample::new(), TagList::add(), Structure::set() and others take more values by reference instead of by value