Rearrange i18n Resource files into a directory

The "app/src/processing/app" directory became mess because there are
a lot of "Resources_*.po" and "Resources_*.properties" files for the
internationalization.

Moved the resource related files into a directory "i18n" under the
"app/src/processing/app" directory.

This is for developers only. No feature changed.

Author: Shigeru KANEMOTO

app/src/processing/app/I18n.java

@@ -35,7 +35,7 @@ static protected void init (String language) {
       if (language != null && language.trim().length() > 0) {
         Locale.setDefault(new Locale(language));
       }
-      i18n = ResourceBundle.getBundle("processing.app.Resources", Locale.getDefault());
+      i18n = ResourceBundle.getBundle("processing.app.i18n.Resources", Locale.getDefault());
 
       PROMPT_YES = _("Yes");
       PROMPT_NO = _("No");