…c#269)

That used to be the case, but getAvailability now automatically uses the
presentation request URL.

Getting the connectionList attribute now returns the same
PresentationConnectionList instance, and only resolves the promise when the
set of presentation controllers is non empty (as things stood, the use of a
Promise was useless).

The commit also drops the step that instructed the receiving user agent to start
monitoring incomping presentation connections if not already started. This is
done in the "starting a presentation" algorithm, no need to repeat it here.

This update should address last Pull Request comments, as well as w3c#281.

Note I ended up introducing two new idioms to talk about the Promise and
PresentationConnectionList singletons, as I believe it improves the readability
of algorithms, and makes the list of objects that user agents need to maintain
explicit.

However, as usual, finding names for things is hard. I came up with
"presentation controllers monitor" and "presentation controllers promise".
It may be possible to find better names. For instance, instead of "monitor", we
could use "manager", "holder", "wrapper", "container".