ClipboardChange event spec: Callout clipboard-read permission is required to listen to clipboardchange

[ragoulik]

Proposing that clipboard-read permission should be called out explicitly as pre-requisite for clipboardchange event to fire:

• From privacy point of view knowing when clipboard has changed deterministically can be considered similar to the act of reading clipboard.
• Since clipboard can be changed by a user action, the information regarding when the clipboard has changed can be considered a user private data. Having the permissions check ensures that this privacy sensitive information is protected by user consent.
• This approach aligns with the security model for other clipboard APIs.