Win32: Revise how the language system works.

The Language menu is eliminated in favour of displaying a NewLanguageScreen on screens that are of type UIDialogScreenWithBackground/UIScreenWithBackground, which comes from a new "Language..." menu item under Game Settings. UIScreen inheriting screens (e.g. GamePauseScreen) still require manual additions.

Addresses https://github.com/hrydgard/ppsspp/issues/4414.