Fixes unespected issues when finding first available target

the previous logic was flawed and would only work for links between
app node <-> device sink node

The main problem observed is when a link that is owned by a WpLink
is removed from the server because one of the linked nodes is gone.
This would cause the APPEARS_ON_REGISTRY flag to go away but the
WpGlobal would still remain in the globals list...

To fix this, forcibly remove the global from the globals list when
it is removed from the registry, even if it is still owned by some
proxy. The proxy at that point is unable to function anyway, because
we make sure to destroy the pw_proxy by removing FEATURE_BOUND when
the global is removed from the registry.

Additionally, ref global->proxy before removing FEATURE_BOUND to
prevent crashing. If the proxy owns the global and the pw-proxy-destroyed
signal causes whoever owns the proxy to drop his reference, _deactivate()
will crash because no-one will be holding a ref to the proxy.

Keeps consistency with PipeWire

The assumption about the global id in wp_registry_prepare_new_global()
was not valid because this function may also be called from the proxy
bound event and this may be before the registry has signalled the
removal of an old global with the same id, for instance

We should instead check this at the point where we expose the global.
If the assertion doesn't hold at that point, we are going to leak an
old WpGlobal, so we must ensure it.

This is to provide easy access to default nodes from the scripts

Do not use WpState at all when use-persistent-storage=FALSE; keep all
the configured default nodes in memory and forget them upon restart

* default endpoints no longer make sense
* follow upstream logic of configured vs effective default nodes
* store node names in json inside the metadata, as upstream does
* add default video source

It looks nicer to have groups of features in 'Features.'
instead of 'Feature.'

It is not needed as si-standard-link can be configured with port contexts

Makes it easier when handling session items in Lua

The get_endpoint_acquisition() vmethod has also been moved to WpSiPortInfo, and
therefore renamed to get_acquisition().

Allows checking if the endpoints are still valid before releasing the
acquisition. Sometimes the endpoints are destroyed before the link, and
releasing acquisition is not needed. There is no way to check that with raw
pointers.