Method add

src/core/layouts/Layout.js:16–20

(key, index = this.children.length)

  }

  add(key, index = this.children.length) {
    const child = this.getObject(key);
    this.children.splice(index, 0, child);
    this.weights.splice(index, 0, 1);
  }

  remove(key) {
    const child = this.getObject(key);