Remove LabelJSO "selected" variable; replace with JSNI methods

In 5af115bde269bcda59bca9c9130c307259d6a4c1, I added a member variable
to a JSO class, which isn't allowed. I've reverted this change and
replaced the functionality with native JSNI methods on a "selected"
object which doesn't exist. (This is cool!)
Represent all labels as instances of LabelJSO

The data structure "modelLabels" in DataManager is still a TreeMap, and
the key of each entry is still the label, but the value of the entry is
now a LabelJSO object.