code: function( splitter ) {
  		var index = lm.utils.indexOf( splitter, this._splitter );
  		
  		return {
  			before: this.contentItems[ index ],
  			after: this.contentItems[ index + 1 ]
  		};
  	}
docs: Locates the instance of lm.controls.Splitter in the array of
  registered splitters and returns a map containing the contentItem
  before and after the splitters, both of which are affected if the
  splitter is moved
  
  @param   {lm.controls.Splitter} splitter
  
  @returns {Object} A map of contentItems that the splitter affects