[fix] KaiTaost display of "next subtitle" was always one behind
Remove AddFileFolder(..) without return value
Merge pull request #2704 from Rawk/use_return_value2

[Cosmetic] Remove AddFileToFolder version without return value
Merge pull request #2705 from Voyager1/fix-nextsub-message

[fix] KaiToast display of "next subtitle" was always one behind
settings: add doxy to all interfaces of the settings system