This is a series of changes necessary to prepare the implementation of remote control
feature:
- XEP-0166: add a `priority` attribute to `ApplicationData`: this is needed when several
applications are working in a same session, to know which one must be handled first.
Will be used to make Remote Control have precedence over Call content.
- XEP-0166: `_call_plugins` is now async and is not used with `DeferredList` anymore: the
benefit to have methods called in parallels is very low, and it cause a lot of trouble
as we can't predict order. Methods are now called sequentially so workflow can be
predicted.
- XEP-0167: fix `senders` XMPP attribute <=> SDP mapping
- XEP-0234: preflight acceptance key is now `pre-accepted` instead of `file-accepted`, so
the same key can be used with other jingle applications.
- XEP-0167, XEP-0343: move some method to XEP-0167
- XEP-0353: use new `priority` feature to call preflight methods of applications according
to it.
- frontend (webrtc): refactor the sources/sink handling with a more flexible mechanism
based on Pydantic models. It is now possible to have has many Data Channel as necessary,
to have them in addition to A/V streams, to specify manually GStreamer sources and
sinks, etc.
- frontend (webrtc): rework of the pipeline to reduce latency.
- frontend: new `portal_desktop` method. Screenshare portal handling has been moved there,
and RemoteDesktop portal has been added.
- frontend (webrtc): fix `extract_ufrag_pwd` method.
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