moved the language stuff to the settings folder

Hello,

I refactored the language class to simplify the management of languages (show open [todo](https://github.com/processing/processing/blob/master/todo.txt#L33)).

The changes:

- [load](https://github.com/voidplus/processing/blob/b642e112f75a0e3125c03b5837050f45f441d39e/app/src/processing/app/Language.java#L61) language from settings folder
- [language.properties](https://github.com/voidplus/processing/blob/i18n_preferences/app/src/processing/app/Language.java#L42) instead of language.txt (it stores the current language and revision number)
- [check](https://github.com/voidplus/processing/blob/i18n_preferences/app/src/processing/app/Language.java#L89) whether the (revision) version has been changed to improve the performance

Happy coding,
Darius