gstreamer: Provide class metadata, pad templates and basetransform...
This is closer to how this works in Python and also how properties/signals work now in the glib bindings.
class_init() only has to be implemented for more special uses now,.
This depends on https://github.com/gtk-rs/gtk-rs/pull/270 which is going to be merged soonish.
Edited by Sebastian Dröge