More flexible translations for "empty" values

The “empty” value actually means any value, ie. any hour, any minute, any month or any day of week, day of month, etc. Translations of “any” for each of these types differ in many languages.

By specifying a language translation for `empty_day_of_month`, one can now customize these translations. If no custom per-type translation is available, the default one for `empty` will be used.

I have not provided default translations for these custom types. I will send another pull request with a proposition to extract translations in a separate file.