When using deeply nested timelines with the `ges:` protocol the
formatters ends up trying to do discovery from the same thread current
discovery happens, leading to infinite freeze as GstDiscoverer can't run
several discoveries at the same time.
By ensuring that when calling `gst_discoverer_discover_uri_async` no
`GstDiscoverer` is set as "thread discoverer" we know that another
discoverer will be created if discovery recurses, effectively removing
the freeze.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5608>
Allowing better control over the way discovery happens and allowing
us to expose a proper API.
This also adds the potential of implementing more multi-threaded
discovery in a clean way in the future.
This allows us to cleanly expose the new
GstDiscoverer::load-serialize-info signal.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3911>