keep=true should work in both ways:
* keep the endpoint with this property linked at all times
* keep other already linked endpoints when linking this one

This is no longer needed since we sort endpoints by role priority
before trying to link them and we link only the highest priority one.
After this sorting, the highest priority endpoint is guaranteed
to be able to link, so _can_link_stream() always returns TRUE.

The proxy takes more roundtrips to get updated and this makes the
policy fail to find a default endpoint at startup, right after
the devices have been discovered

This also removes the weak ref on the session, since the object
manager internally maintains a reference on the objects that
it manages.

This is now done by the config endpoint module

to make it more usable and compatible with g_idle_add

and drop the WpBaseEndpoint controls entirely

This keeps track of the default endpoint and selects a default
based on endpoint priorities when devices are discovered

This is useful to be able to call export() from a module init
function, which is called before the core is connected

Clear self->pending_endpoint before sending the done signal,
because the signal will trigger a new endpoint to be added in the
unit test and this may happen before g_clear_object is called,
in which case adding the new endpoint fails

These are set by the server on the global properties set

heavily based on the WpSession implementation