message: Convert `DividerMenuItem` over to lit-html
diff: /*
  - * Copyright 2018 Simon Edwards <simon@simonzone.com>
  + * Copyright 2018-2020 Simon Edwards <simon@simonzone.com>
  *
  * This source code is licensed under the MIT license which is detailed in the LICENSE.txt file.
  */
  import { WebComponent } from 'extraterm-web-component-decorators';
  +import { html, render } from "extraterm-lit-html";
  import * as ThemeTypes from '../../theme/Theme';
  -import { TemplatedElementBase } from './TemplatedElementBase';
  +import { ThemeableElementBase } from '../ThemeableElementBase';
  -const ID_CONTAINER = "ID_CONTAINER";
  /**
  * Divider line menu item for use inside a context menu.
  */
  @WebComponent({tag: "et-divider-menu-item"})
  -export class DividerMenuItem extends TemplatedElementBase {
  -
  - static TAG_NAME = 'ET-DIVIDER-MENU-ITEM';
  +export class DividerMenuItem extends ThemeableElementBase {
  constructor() {
  - super({ delegatesFocus: false });
  + super();
  + this.attachShadow({ mode: "open", delegatesFocus: false });
  + this.updateThemeCss();
  + this._render();
  }
  - protected _html(): string {
  - return `
  - <div id='${ID_CONTAINER}'>
  - <hr>
  - </div>`;
  + protected _render(): void {
  + const template = html`${this._styleTag()}
  + <div id='ID_CONTAINER'><hr></div>`;
  + render(template, this.shadowRoot);
  }
  protected _themeCssFiles(): ThemeTypes.CssFile[] {
  