Win32: Revise how the language system/menu 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.

Not that it really needs any screenshots, but..

![Screenshot 01](https://i.minus.com/ibcn6tURt3pnKH.jpg)

![Screenshot 02](https://i.minus.com/iMB3frkT0wpvL.PNG)