Specification: https://w3c.github.io/uievents/#legacy-interface-UIEvent

We have an old implementation in Servo that's gated behind the dom_mouse_event_which_enabled preference, back from when it was quite underspecified and its future was uncertain. This issue is tracking the work required to ship this property enabled by default.

Initial work:

• Move the which attribute from MouseEvent.webidl to UIEvent.webidl
• Move the Which implementation from MouseEvent to UIEvent
• Replace the logic in Which with a getter for a member initialized from UIEvent::new_inherited, and pass the appropriate value from MouseEvent::new_inherited
• Rename the preference to dom_uievent_which_enabled
• Add the preference to the experimental web platform features and update test expectations (https://book.servo.org/hacking/testing.html#running-web-platform-tests-on-your-github-fork and https://book.servo.org/hacking/testing.html#updating-web-platform-test-expectations)

Followup work:

• Add the which member to UIEventInit and update UIEvent's constructor appropriately
• Make KeyboardEvent::new_with_proto set the UIEvent.which property to keyCode (https://w3c.github.io/uievents/#dom-uievent-which)

Finally:

• Set the default preference value to true