constructor () {
    // this will hold the <g> element that contains all selector rects/grips
    this.selectorParentGroup = null

    // this is a special rect that is used for multi-select
    this.rubberBandBox = null

    // this will hold objects of type Selector (see above)
    this.selectors = []

    // this holds a map of SVG elements to their Selector object
    this.selectorMap = {}

    // this holds a reference to the grip elements
    this.selectorGrips = {
      nw: null,
      n: null,
      ne: null,
      e: null,
      se: null,
      s: null,
      sw: null,
      w: null
    }

    this.selectorGripsGroup = null
    this.rotateGripConnector = null
    this.rotateGrip = null

    this.initGroup()
  }
Sets up properties and calls `initGroup`.