* 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`.
We prefer to prevent using chained assignment (`T a = b = c = T();`) as this
can lead to confusing code and subtle bugs.
According to https://en.wikipedia.org/wiki/Assignment_operator_(C%2B%2B), C++
allows any arbitrary return type, so this is standard compliant.
This could be re-assessed if/when we have an actual need for a behavior more
akin to that of the C++ STL, for now this PR simply changes a handful of
cases which were inconsistent with the rest of the codebase (`void` return
type was already the most common case prior to this commit).
