When a pw_global is removed on the server (by pw_registry_destroy() or other
means), it triggers the proxy removed & the registry global_remove callbacks,
but it does not necessarily destroy the pw_proxy.

For client proxies, we were previously destroying them by unrefing the WpProxy
in wp_global_rm_flags(), since the global was not "owned" by the WpProxy.

For impl proxies, we were not doing anything, as we expected that it would
only be removed from the registry if the local WpProxy was destroyed first.
This is not always the case, though, as the server or another client may
request to destroy this proxy with pw_registry_destroy()

Now we always destroy the pw_proxy as soon as it is removed from the registry,
no matter if it was a client or an impl proxy. If it was an impl proxy,
the WpProxy will continue to live and it's up to the code that created it
to handle the "pw-proxy-destroyed" signal and do something meaningful.
If it was a client proxy, the global will still unref the WpProxy right after
destroying the pw_proxy and there is no change in behavior.

Useful to destroy links and endpoint-links

streams should be provided externally,
the user may choose to create any kind of stream

This won't work with WpSpaPod, obviously...

It's not well-defined; we'll come back to that later

This reverts commit 4e8ae293.

Needed when using audiotestsrc with an adapter

- Use similar code for consistency
- Add changed signals everywhere
- Port to the new object-manager API

signal handlers expect FEATURE_BOUND to be set