Make --doctool locale aware

* Adds `indent(str)` to `String`:
    * Indent the (multiline) string with the given indentation.
    * This method is added in order to keep the translated XML correctly
      indented.
* Moves the loading of tool/doc translation into
  `editor/editor_translation.{h,cpp}`.
    * This will be used from both `EditorSettings` and the doc tool from
      `main`.
* Makes use of doc translation when generating XML class references, and
  setup the translation locale based on `-l LOCALE` CLI parameter.

The XML class reference won't be translated if `-l LOCALE` parameter is
not given, or when it's `-l en`.

main/main.cpp

@@ -83,6 +83,7 @@
 #include "editor/doc_tools.h"
 #include "editor/editor_node.h"
 #include "editor/editor_settings.h"
+#include "editor/editor_translation.h"
 #include "editor/progress_dialog.h"
 #include "editor/project_manager.h"
 #ifndef NO_EDITOR_SPLASH
@@ -1937,7 +1938,6 @@ Error Main::setup2(Thread::ID p_main_tid_override) {
 	}
 
 	_start_success = true;
-	locale = String();
 
 	ClassDB::set_current_api(ClassDB::API_NONE); //no more APIs are registered at this point
 
@@ -2049,6 +2049,11 @@ bool Main::start() {
 		// Needed to instance editor-only classes for their default values
 		Engine::get_singleton()->set_editor_hint(true);
 
+		// Translate the class reference only when `-l LOCALE` parameter is given.
+		if (!locale.is_empty() && locale != "en") {
+			load_doc_translations(locale);
+		}
+
 		{
 			DirAccessRef da = DirAccess::open(doc_tool_path);
 			ERR_FAIL_COND_V_MSG(!da, false, "Argument supplied to --doctool must be a valid directory path.");