Move target attributes to event['target']

[rmorshea]

Current Situation

Currently attributes of the event target are simply assigned to the top-level of the event object. In retrospect this was a mistake because there are different kinds of targets (e.g. currentTarget and relatedTarget) in addition to the possibility of conflicts with the event's own attributes.

Proposed Changes

Create new keys in event information for "currentTarget", and "relatedTarget" where target element attributes will be stored. In addition event information for target will be moved under a new "target" key.

Implementation Details

Since attributes of target are currently available at the top-level of the event dictionary, to avoid an immediate backwards incompatible change, we will create an EventProxy object that wraps the original event data and aliases any missing keys to event['target'][key]. When the EventProxy object needs to alias a key in this way, it will produce a warning that attributes of target have been moved to the "target" key.