Make editor use translation domains

How editor plugins use this feature:
1. Pick a unique translation domain name.
2. `_enter_tree()`: load translations into that translation domain.
3. Call `set_translation_domain()` for its root UI node.
4. `_exit_tree()`: remove that translation domain.

Plugins can also set the translation domain to `godot.editor` for
nested nodes that should use editor translations. `EditorFileDialog`
automatically does this.

editor/editor_settings.cpp

@@ -1210,6 +1210,8 @@ fail:
 
 void EditorSettings::setup_language() {
 	String lang = get("interface/editor/editor_language");
+	TranslationServer::get_singleton()->set_locale(lang);
+
 	if (lang == "en") {
 		return; // Default, nothing to do.
 	}