Make the leaks tracer happy, allowing us to detect actual leaks.

Fix #341

Needed so we can use the leaks tracer.

We can't use gtk::main_quit() as the loop is handled by the
GtkApplication.

Since https://github.com/gtk-rs/gtk-rs/pull/447, supposedly the native
function is never returning a `NULL` `Application` hence it makes little
sense to handle an "unreachable" error here.

Timeouts now use std::time::Duration instead of plain integers.

This code rework the examples to use the new 2018 edition and also
rework the code to avoid using unnecessary 'extern crate' calls.

The 'use extern crate gstreamer as gst', as well as the other gstramer
related crates, were kept, otherwise we'd need to do it on 'Cargo.toml'
but it would make it more difficult to figure out the respective crate
name.

Fixes #216

See #171

... in function signatures.

These breaking changes aim at improving usability by allowing users
to take advantage of Rust error management features sur as `ok_or`,
`map_err`, `expect` and the `?` operator. See the `examples` and
`tutorials` to get an idea of the impacts.

Added verbose documentation to all of the repository's examples.

These are now all leak-free.

Mathieu Duponchelle authored Feb 26, 2018 and Sebastian Dröge committed Feb 26, 2018

This lets us avoid #ifdef forests

François Laignel authored Jan 29, 2018 and Sebastian Dröge committed Jan 29, 2018

Generic methods for events, messages and queries can now be invoked from the concrete type.

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.

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

And use them in the examples.

And use those in the examples instead of blanket * imports