This is mostly useful when using the adapter with audiotestsrc,
which supports an infinite number of channels. By specifying
a preference, we can limit it to the number of channels that we want.

* there is just one link, no need for a list
* warn if something goes wrong and don't assert

streams should be provided externally,
the user may choose to create any kind of stream

This won't work with WpSpaPod, obviously...

It's not well-defined; we'll come back to that later

This reverts commit 4e8ae293.

Needed when using audiotestsrc with an adapter

This is a simpler endpoint implementation that does not require
any node setup. It should be good to use with test nodes (fakesrc/fakesink),
V4L2 nodes, and maybe others in the future.

+ add the useful _find_proxy() method

+ expose the export transition in the session item class
+ make the export-related flags immutable
+ add an export error flag
+ update and improve documentation

+ replace calling execute_step(..., STEP_ERROR) with rollback
+ implement deactivate internally using rollback

This unifies deactivation steps, which are common between deactivate()
and calling execute_step() with WP_TRANSITION_STEP_ERROR at the
end of a failed activation transition.