PipeWire headers were recently cleaned up to reduce the number of
included headers. This leads to a number of functions and types not
being included when needed in wireplumber.

When the audioconvert starts, it emits 2 ports, but when we set the PortConfig,
it removes them and re-creates them. Previously, the stream class would not
remove the old port proxies from the list and therefore they existed twice.

It is also necessary here to store the proxies earlier, when they are added,
instead of when they are augmented, so that we can ensure they are removed.
Previously we would hit an issue where:
 - port proxy is added, augmented
 - augment completes but the GTask wants to complete asynchronously:
   it stores a ref on the proxy and adds an idle source
 - server removes the proxy, we delete it from the core's list
 - the GTask now calls the augment callback, which stores the (removed)
   proxy on the stream's port_proxies list...

Running audioconvert in merge+split mode is the only way to make this work with
the adapter, since the adapter does not support passing multiple channels on
a single port right now, and if it does at some point, it will be without a
mixing node on the port, which means we will not be able to mix multiple
audioconvert nodes on the same adapter. In the future we need to consider
writing a lighter volume node with multiple channels support to replace
audioconvert.

The new linking algorithm now takes into account the channel positions and makes
sure to link the correct channels together. Also, it avoids passing the port
proxies inside the GVariants, thus making the algorithm a bit more generic
and easier to unit test.

In practice we always create a remote and connect to pipewire.
Any other scenario is invalid, therefore, it is not justified
to be confused with so many classes for such small functionality.
This simplifies a lot the modules code.

Also, this commit exposes the pw_core and pw_remote objects
out of WpCore. This is in practice useful when dealing with low-level
pw and spa factories, which are used in the monitors. Let's not
add API wrappers for everything... Bindings will never use this
functionality anyway, since it depends on low level pipewire C API.