endpoint.description is now the user-friendly string
we want the endpoint name to be machine-friendly because it's
the identifier that we store in m-session-settings

- 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..

m-device-activation: use wp_core_timeout_add_closure instead of wp_core_timeout_add in reserve node API

This allows dbus requests to be done inside the task callback

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

This module suspends a node if idle for at least 3 seconds

* 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