Previously, when using `switch_to_locale()` all current loaded text domains were unloaded and added to the `$l10n_unloaded` global. This prevented the just-in-time loading for text domains after a switch. The just-in-time loading was also only possible if the translations were stored in `WP_LANG_DIR`. Both issues have been fixed.
* Adds `WP_Textdomain_Registry` to keep track of the language directory paths for all plugins and themes.
* Updates all `load_*_textdomain()` functions to store the path in `WP_Textdomain_Registry`.
* Adds `$reloadable` parameter to `unload_textdomain()` to define whether a text domain can be loaded just-in-time again. This is used by `WP_Locale_Switcher::load_translations()`.
* Extends `_load_textdomain_just_in_time()` to also support text domains of plugins and themes with custom language directories.
* Fixes the incorrect `test_plugin_translation_after_switching_locale_twice()` test which should have catch this issue earlier.
* Adds a new test plugin/theme to test the loading of translations with a custom language directory.
* Deprecates the now unused and private `_get_path_to_translation()` and `_get_path_to_translation_from_lang_dir()` functions.
Props yoavf, swissspidy, dd32, ocean90.
See #26511.
Fixes#39210.
Built from https://develop.svn.wordpress.org/trunk@49236
git-svn-id: http://core.svn.wordpress.org/trunk@48998 1a063a9b-81f0-0310-95a4-ce76da25c4cd
