settings: add explicit button font

Fonts for menu foot and button font are not not necessarily the same thing.

TODO: locales

src/system/settings.h

@@ -688,6 +688,7 @@ struct SNeutrinoSettings
 		FONT_TYPE_MOVIEBROWSER_INFO,
 		FONT_TYPE_SUBTITLES,
 		FONT_TYPE_MESSAGE_TEXT,
+		FONT_TYPE_BUTTON_TEXT,
 		FONT_TYPE_COUNT
 	};