Make the code to export objects more generic. Make it possible for
modules to register a type to export.
Make the client-node also able to export plain spa_nodes.
Let the remote signal the global of the exported object if any. We can
then remote the (unused) remote_id from the proxy.
Remove some events from the remote that we can find on the core_proxy.
Use the core_proxy to get to the done and info events.
Remove pw_remote_get_core_info(), we don't need this anymore now that
we don't listen for the event and the user can keep track of this
herself.
