[TallblokeUK]

Re-applies 004a4f6, which #564 undid along with the other commits from 21 September. Settings_Fields.php on core-beta and at v4.0.0-beta.3 is identical to how it stood before that fix.

Reading any setting built every settings field, translated labels included. The shortcode handler reads a setting on the_posts, which fires for any post query, so a query made while plugins are still loading requested translations before WordPress allows them. WordPress reports that as _load_textdomain_just_in_time was called incorrectly, and with debug display on the notice is printed ahead of the headers.

Reported against 3.10.0 and 3.10.2: https://wordpress.org/support/topic/translation-loading-too-early-in-code-snippets-3-10-2/

Verification

• Reproduced on the released 3.10.2 and on core-beta with a non-English locale and a post query on plugins_loaded; the notice traces to Settings_Fields::init_fields() via Shortcodes::enqueue_highlighting(). With this change the same request raises nothing, and the settings screen still renders.
• New tests read a setting, and run a post query, and assert nothing is translated in the plugin's text domain. Each runs in its own process, because the field definitions are built once per process. Both fail without the change, alone and within the full suite.
• Full PHPUnit suite (355 tests) and lint:php pass.