constructor(sNodeProperty, fnGetKeyFilter) {
			this.fnGetKeyFilter = fnGetKeyFilter;
			this.sNodeProperty = sNodeProperty;
		}
Constructor for a new _TreeState.
The tree state is only kept if a <code>sNodeProperty</code> is given.

@param {string} [sNodeProperty] - The path to the node id property
@param {function(object):string} fnGetKeyFilter
  A function to calculate a node's key filter

@public