saveWidths() {
    let columns = {};
    let totalWidth = this._columns.reduce((sum, { width }) => sum + width, 0);
    this._columns.forEach(({ id, width }) => {
      columns[id] = width / totalWidth;
    });
    let saved = this.storage.getItem(this.getStorageKey()) || {};
    saved.columnWidths = columns;
    this.storage.setItem(this.getStorageKey(), saved);
  }
Uses local storage to cache the current column widths. A specific
table's widths will remaing unchanged anyime the inspector is opened again.

The stored widths are percentages so that they remain independent of a table's
width.

@method saveWidths