Need `Since` markers for various enum/flags/boxed `get_type()` functions
E.g. gst_video_overlay_format_flags_get_type()
was added in 1.16, but the corresponding enum was added long long ago. We need to add the correct Since
markers so that bindings can correctly handle versioning.
The correct versions can be taken from the Debian packaging:
- https://salsa.debian.org/gstreamer-team/gstreamer1.0/blob/master/debian/libgstreamer.symbols
- https://salsa.debian.org/gstreamer-team/gst-plugins-base1.0/blob/master/debian/libgstreamer-plugins-base.symbols
- https://salsa.debian.org/gstreamer-team/gst-plugins-base1.0/blob/master/debian/libgstreamer-gl.symbols
To solve this we should probably generate a static gtk-doc header file for all these, and then ideally have a test that checks that this header file stays up to date (I'll complain when updating the Rust bindings, but something automatic would be better).
This also affects GStreamer core.