getDraggableElementsForContainer(container) {
    const allDraggableElements = container.querySelectorAll(
      this.options.draggable,
    );

    return [...allDraggableElements].filter((childElement) => {
      return (
        childElement !== this.originalSource && childElement !== this.mirror
      );
    });
  }
Returns draggable elements for a given container, excluding the mirror and
original source element if present
@param {HTMLElement} container
@return {HTMLElement[]}