saveVisibility() {
    let saved = this.storage.getItem(this.getStorageKey()) || {};
    saved.columnVisibility = this._columnVisibility.reduce(
      (obj, { id, visible }) => {
        obj[id] = visible;
        return obj;
      },
      {},
    );
    this.storage.setItem(this.getStorageKey(), saved);
  }
Caches which columns are visible and which ones are hidden.
Uses local storage. Visibility settings will remain unchanged
whenever the inspector is used another time.

@method saveVisibility