initSelectablesOnGestureStart = function() {
    var self = this,
        $this = $(this),
        data = $this.data(pluginName);

    var selectables = [];
    $this.find(data.options.filter).each(function() {
      var selectable =
        (data.options.positioner || defaultPositioner).call(self, this);
      selectable.element = this;
      selectable.selected = false;
      selectables.push(selectable);
    }).removeClass(pluginName + '-selected');
    data.selectables = selectables;
  }
Initializes all the selectable elements' when the selection gesture
starts. This includes caching their current position and clearing any
previous selection.