This singleton class is an editor state encapsulation object. Every part
of the editor could store its state-holder here and it will be automatically
persistent across restarts. It is intended for any state informations
that are not "Settings", like the contents of the input field histories,
persistent, named bookmarks or so.
The implementation is just like a HashMap indexed by state-holders' names.
Typical usage is myState = EditorState.get( MY_STATE_NAME );
There is no support for state change notifications, but the inserted
value objects could be singletons as well and could do its own notifications.