Add bindings for libgstpbutils

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

There are multiple steps here:

Add a Gir_Gst*.toml and autogenerate all relevant types, see https://github.com/gtk-rs/gir/
Make sure the autogenerated code is correct, add manual code as needed
Write manual bindings for anything that can't be automatically generated (macros, miniobjects, ...)
Add src/lib.rs for re-exporting, Cargo.toml, etc

Depends on https://github.com/sdroege/gstreamer-rs/issues/7 https://github.com/sdroege/gstreamer-rs/issues/5 https://github.com/sdroege/gstreamer-rs/issues/4

Edited Apr 25, 2021 by Sebastian Dröge

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