Allows implementing device specific endpoint creation logic, which is currently
needed for bluetooth endpoints as they need to be created differenly.

* do not copy the full alsa node properties set
* use a node description that makes the nodes look better in JACK
* use . instead of / as a separator for the node.name, like elsewhere
* add audio.convert spa lib association in the tests that use si-convert;
  previously it used to work because library.name was present in the
  properties copied from the adapter (and it so happens that the adapter
  lives in the audioconvert spa plugin as well ...)

and do not allow writing to wrapped pw_properties,
since they are const now

- make it a GObject so that it can emit its own signals
and so that it can be shared between multiple proxies
- share the WpProps instance between endpoints, endpoint-streams
and their underlying nodes
- introduce the concept of the caching mode that redirects _set
to _set_param of the proxy that actually has the props; this allows
shared WpProps to actually set changes on the correct proxy
in a transparent way
- change methods to consume the ref of the pod and reflect that
also on wp_proxy_set_prop()
- refactor the export process on endpoints & endpoint-streams
so that they always get all the required features (info, props, bound)
and make it async so that we can take time to prepare the underlying
node to have FEATURE_PROPS
- update the props & endpoint unit tests, bringing back all the
checks that the endpoint unit test used to have

m-config-static-nodes: remove unneeded 'conected' callback and create static nodes when the plugin is activated

* Use a direction instead of a string prop id
* Emit default-endpoint-changed again
* Get rid of the useless virtual methods

+ rename FEATURE_CONTROLS to FEATURE_PROPS
+ add accessor for the standard spa_param_info (info->params)
+ hide the low-level params API that nobody uses

+ use a more correct naming scheme to match the one from Spa
+ remove the default video source, since we now use a different
 session for video

Features are flags, therefore we must NEVER use them without a shift,
otherwise bad mistakes happen, like the previous mistake of declaring
WP_SESSION_FEATURE_LINKS as the number after WP_SESSION_FEATURE_ENDPOINTS,
which ended up being (WP_SESSION_FEATURE_ENDPOINTS | WP_PROXY_FEATURE_PW_PROXY)
and it was always becoming available together with the ENDPOINTS feature.