Integrate new subclassing infrastructure based on new glib-rs API

This now makes use of the new subclassing infrastructure that was added to glib-rs the other day. Most of the code is based on what's currently in gst-plugin-rs/gst-plugin but simplified a lot. gst-plugin-rs/gst-plugin will also disappear once this is completely done and all plugins are ported over (see WIP at https://gitlab.freedesktop.org/slomo/gst-plugin-rs/tree/subclass).

Overall this simplifies the code a lot, is easier to understand and has lower overhead.