Implement Apple Translate

[Havhingstor]

Instead of just their instance and DeepL, the user has now the option between their instance, DeepL and Apple Translate. The user's previous choice (DeepL on / off) is reflected in the first value of the new setting and the old always_use_deepl App-Storage Key is deleted. The option is only available on iOS 17.4 or later and not on Mac or Vision Pro. If the user is on an unsupported platform but the option to use Apple Translate is set, it's reset to use the instance.

This also includes a couple of fixes that aren't necessary for Apple Translate, but related:

• The background of the "Auto detect language" button is now consistent with all the other rows
• All loaded translations are removed if the translation type is switched
• A warning is shown to the user if they've switched away from using DeepL, but there's still an API key stored
• The spelling of "DeepL API Key" is unified, previously the "Key" was sometimes lowercase.

Tested on iPhone, iPad, Mac & visionOS Simulator