This scales better as there will only be only such data instead of two
or more when having deeper class hierarchies with multiple Rust
elements, and also makes it unnecessary to use a special instance struct
so the default works well.
This is closer to how this works in Python and also how properties and
signals work now in the glib bindings.
class_init() only has to be implemented for more special uses now.
The 'failure' crate has been stale for quite some time and better
alternatives has been developed since its introduction. We choose the
'anyhow' and 'derive_more' to replace it.
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.