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.