Settings: Create a setting to disable subtitles by default

Addresses #865 (closed).

Changes here create a new option in Settings to add the ability to disable subtitles upon loading new media.

Other discussion

With the motivation of having the more-user-friendly text ("On; use default track") to help clarify the state of the setting, I had initially trialled this with a switch in the app, and a multi-value setting only in Settings.app, but would still define the multi-value setting in-app so that the app's architecture picks up on the changes in the subtitle (with the `localeDictionary). Additionally, the user would able to more-easily toggle the state while inside the app.

I ended up figuring that it may be better for both the Settings.app and the in-app settings to maintain the same UX, and to not introduce an oddity in the way the Settings screen is set up.

Unfinished

With English being my only strong language, I don't know how I should go about the localisation for other languages. Could somebody help me on that front?

Screenshots

	Top Level - Selected On	Top Level - Selected Off	Selection Menu
Settings.app
In-app Settings