Add subclassing support for GstVideoDecoder

See https://gstreamer.freedesktop.org/data/doc/gstreamer/head/gst-plugins-base-libs/html/gst-plugins-base-libs-GstVideoDecoder.html

It probably makes sense to first wrap more parts libgstvideo in gstreamer-rs so that things like GstVideoCodecFrame and GstVideoInfo/Frame can possibly be directly used without writing manual bindings. https://github.com/sdroege/gstreamer-rs/issues/5

As part of this should also be a simple decoder element. Maybe just a minimal element that takes e.g. a BMP image and decodes it (or rather parses the header and throws away the header).