I18N: Introduce WP_Textdomain_Registry to store text domains and their language directory paths.

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.

git-svn-id: https://develop.svn.wordpress.org/trunk@49236 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/admin/includesPlugin.php

@@ -14,7 +14,7 @@ class Tests_Admin_includesPlugin extends WP_UnitTestCase {
 			'Description' => 'This is not just a plugin, it symbolizes the hope and enthusiasm of an entire generation summed up in two words sung most famously by Louis Armstrong: Hello, Dolly. When activated you will randomly see a lyric from Hello, Dolly in the upper right of your admin screen on every page. <cite>By <a href="http://ma.tt/">Matt Mullenweg</a>.</cite>',
 			'Author'      => '<a href="http://ma.tt/">Matt Mullenweg</a>',
 			'AuthorURI'   => 'http://ma.tt/',
-			'Version'     => '1.5.1',
+			'Version'     => '1.7.2',
 			'TextDomain'  => 'hello-dolly',
 			'DomainPath'  => '',
 		);