Add secondary sub menu to extension button popup

[gmertes]

I would like to add a backup secondary language list to the settings page. So that if Netflix ever breaks the layout again, we can fall back to this menu in the settings page and the extension can keep working.

If possible, the backup menu should only be visible if the regular injection in the web-player fails (bonus: show a warning on screen to make the user aware of this). It may not be possible to check this reliably (because the secondary menu is only injected when the subtitle menu is visible). As an alternative, the menu should be small and not add bulk to the settings page. Layout should be kept as simple and small as possible, ideally just a select box with languages.

PR welcome.

[Rachel-Tan0]

Netflix Dual Subtitles - Subtitle Translator can do it.