* 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`.
Haoyu Qiu