this allows slightly easier testing while every client uses node.target
but it's easier to find the endpoint id than the node id
through `wpctl status`

- fall back to the default logic if node.target is set but it doesn't exist
- use the default endpoint if it is within the [target-endpoint] matches;
  this way we can use a config file to override just the stream name,
  for instance, while still respecting the default endpoint
- prefer use of the lookup methods instead of manual iterations

previously the default endpoint was not respected because our config
files have a [target-endpoint] table..

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

+ 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