Events and sundry

Useful forward motion for DOM events

• dispatch missing common DOM events (mousemove, mousedown, mouseup, click) based on Servo events dispatched by the compositor task (see send_mouse_event)
  • double click events?
  • https://github.com/mozilla/servo/pull/1866 adds centralized click dispatching and handling, but it's mostly orthogonal/rebaseable without much effort
  • once the above is merged, implementing mouseenter/mouseleave events and integrating hover styling (see MouseOverQuery) support would be a nice improvement
• touch events for android - probably a similar setup to the way we deal with mouse events in the compositor