Timeline "isSelected" and "unselectItem" don't work with clusters

[CASandmann]

These methods along with setSelection([]) behave incorrectly when the user has clicked a cluster. These methods check for (this.selection && this.selection.index !== undefined) but if the most recent click was on a cluster, selection looks like {cluster: 0} which doesn't pass that condition.

This leads isSelected(clusterIndex) to return false and setSelection([]) and unselectItem([]) to be a no-op and the next time the user calls getSelection() it will return the cluster again. The current workaround is to set timeline.selection.index to a value, but that definitely isn't intuitive.