removeChildren_() {
    const nodes = this.el.childNodes;
    let i = nodes.length;

    while (i--) {
      const node = nodes[i];

      if (!Slide.isCandidate(node)) {
        this.el.removeChild(node);
      }
    }
  }
Removes all children elements inside of the main container that are not
eligible to be a Slide Element.
@private