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

to avoid name clashing with the upcoming WpEndpoint interface
that is going to be the common interface of Wp{Proxy,Exported}Endpoint

This avoids hearing mono audio only on the left speaker

the _can_link function was previously assuming that the target
stream is always specified in the configuration file

So that we can use media.role to specify the stream

Big API break happens after this commit and we cannot
keep up right now.

The role defined in the endpoint-link configuration files must have higher
priority than the one set in the endpoint. If none of them are set, the config
policy fallbacks to the lowest one defined in the streams configuration file

Currently all it does is that it prints the properties
of all device nodes (Audio/Source & Audio/Sink), which
is useful for editing the configuration files

Otherwise, if the object manager is destroyed while a sync is in progress,
we get an invalid 'self' pointer on the callback later, which is being
called regardless

There is a bit more work that should be done in the core to avoid leaking
this ref in case pipewire disconnects before the sync is completed