Clarify "fire a blank event" for "capturedzoomlevelchange"

[tidoust]

The spec current defines the oncapturedzoomlevelchange event handler as: "The user agent MUST fire a blank event on this EventHandler ...", but:

• What is a "blank event"? In particular, what interface does the event use? Event? That should be specified, at least implicitly by linking back to "fire an event".
• An event does not fire on an event handler. It fires at a target (an instance of some interface). Here, that would be the underlying CaptureController instance.

Ideally, the spec would rather say something like:

Given a {{CaptureController}} |controller|, the user agent MUST [=fire an event=]
named `capturedzoomlevelchange` at |controller| whenever the
|controller|.<a data-cite="!screen-capture/#dfn-source">[[\Source]]</a>'s
[=zoom level=] changes.

... and the event handler would rather be defined as:

An [=event handler IDL attribute=] whose [=event handler event type=] is
`capturedzoomlevelchange`.

[eladalon1983]

Happy to affect these changes after the holidays. Thanks for suggesting them.