add (window) {
    const { id: windowId } = window
    this._windows.set(windowId, window)

    if (!this._appMenu.has(windowId)) {
      this._appMenu.addDefaultMenu(windowId)
    }

    if (this.windowCount === 1) {
      this.setActiveWindow(windowId)
    }

    window.on('window-focus', () => {
      this.setActiveWindow(windowId)
    })
    window.on('window-closed', () => {
      this.remove(windowId)
      this._watcher.unwatchByWindowId(windowId)
    })
  }
Add the given window to the window list.

@param {IApplicationWindow} window The application window. We take ownership!