Proper i18n solution for "Files for direct translation"

[RanYanivHartstein]

The Codex page ​Files for Direct Translation lists all the core files that translators have to edit directly for a complete localization.

These are mostly error messages in files that load before gettext is initialized. Some of them are loaded before wp_config.php and therefore can't tell yet what the proper locale is. One gets loaded when wp_config.php is *missing*, so there is no way of knowing what the proper locale is (this is discussed in ticket #6132).

A system needs to be put in place to allow localizing these strings without editing core files, or at least of editing just one core file in a specific location. This should be independent of the gettext system. For e.g., a single file place in the root folder that sets these strings as variables to be used in the various other files that need them.

The patch above wraps strings, needing direct translation, in special comments.

Using the ​not-gettexted.php tool these strings can be extracted in POT files or replaced with translations from a MO file.

(In [8168]) Mark string that cannot be gettexted. Props nbachiyski. fixes #7074