This patch removes the mutable API to event handling (Nodes no longer have a events object with which to add listeners).

Instead, a new (non-Render) Node is introduced: EventTarget. This node represents a location in the Effen tree which can handle events as they bubble.

Note that this also changes the implementation to use event delegation (one set of listeners at the sky.document level) rather than direct listeners on leaf nodes.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1019633004