TrackballControls should extend EventDispatcher (threejs)

Please see
https://github.com/mrdoob/three.js/blob/master/examples/js/controls/Trac
kballControls.js#L611

The prototype of THREE.TrackballControls is
THREE.EventDispatcher.prototype.
Merge pull request #3122 from tomtung/master

TrackballControls should extend EventDispatcher (threejs)