Kill onbeforecopy, onbeforecut and onbeforepaste events

[hallvors]

These events don't do a good job at fulfilling their use cases, have pretty much no good implementation and are confusingly named (making people think they resemble an event like beforeInput).

We can watch w3c/editing#93 to see if the editing group comes up with a generic "let the browser know that command x is enabled/disabled" feature we can use instead.