Add missing features to Source trait
There are various other features (specifically virtual methods and their arguments) that GstBaseSrc has and we don't wrap yet. This should probably come with adding support for libgstbase in gstreamer-rs: Might make sense to do this first: https://github.com/sdroege/gstreamer-rs/issues/7
Most of these features can be wrapped basically 1-1: https://gstreamer.freedesktop.org/data/doc/gstreamer/head/gstreamer-libs/html/GstBaseSrc.html