Add event for trackedEntity changes.

[emackey]

This PR consists of 99.5% recycled code lines from @hpinkos selectedEntityChanged event in #5043.

[emackey]

var viewer = new Cesium.Viewer('cesiumContainer');

viewer.dataSources.add(Cesium.CzmlDataSource.load('../../SampleData/simple.czml'));
viewer.camera.flyHome(0);

viewer.selectedEntityChanged.addEventListener(function(entity) {
    if (entity !== undefined) {
         console.log('selected ' + entity.id);
    } else {
         console.log('de-selected.');
    }
});

viewer.trackedEntityChanged.addEventListener(function(entity) {
    if (entity !== undefined) {
         console.log('tracked ' + entity.id);
    } else {
         console.log('Stopped tracking.');
    }
});

[hpinkos]

Looks great, thanks @emackey!