How to filter available screens according to the content being presented (issue #9)

All,

This issue is all about ensuring that the user is not presented with a
"present" icon which, when clicked, does nothing because there are in fact
no available screens supporting the content.

The issue suggests one approach, whereby the UA can be instructed to filter
the available screens to those supporting specific provided URLs.

Another approach would be to provide a new method:

Promise<bool> canRequestSession( DOMString url );

This would take exactly the same parameters as requestSession, but rather
than prompting the user it would return a prediction of whether there would
be any screens to prompt the user with. The application could avoid showing
the "present" icon if there are in fact no suitable screens.

...Mark