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`.

core/string/ustring.h

@@ -356,6 +356,7 @@ public:
 
 	String left(int p_pos) const;
 	String right(int p_pos) const;
+	String indent(const String &p_prefix) const;
 	String dedent() const;
 	String strip_edges(bool left = true, bool right = true) const;
 	String strip_escapes() const;