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`.

tests/core/string/test_string.h

@@ -1132,6 +1132,25 @@ TEST_CASE("[String] c-escape/unescape") {
 	CHECK(s.c_escape().c_unescape() == s);
 }
 
+TEST_CASE("[String] indent") {
+	static const char *input[] = {
+		"",
+		"aaa\nbbb",
+		"\tcontains\n\tindent",
+		"empty\n\nline",
+	};
+	static const char *expected[] = {
+		"",
+		"\taaa\n\tbbb",
+		"\t\tcontains\n\t\tindent",
+		"\tempty\n\n\tline",
+	};
+
+	for (int i = 0; i < 3; i++) {
+		CHECK(String(input[i]).indent("\t") == expected[i]);
+	}
+}
+
 TEST_CASE("[String] dedent") {
 	String s = "      aaa\n    bbb";
 	String t = "aaa\nbbb";