Make access to clipboard events a per-website permission (like canvas read)

Old Description:

While reading through various about:config security hardening guides, I found several bad default values for the Tor Browser:

• dom.event.clipboardevents.enabled = false
  • Disable that websites can get notifications if you copy, paste, or cut something from a web page, and it lets them know which part of the page had been selected.

(This issue was split from https://trac.torproject.org/projects/tor/ticket/27059)

Trac:
Username: floweb

Sometimes clipboard events are for good, sometimes for bad. User's should be able to configure as they see fit per website ( see: #27882 (comment 2819119) )