`convertBiDiStringByLines` calls the BiDi algo for each line in isolation,
and returns a joined result.
That's needed to support BiDi in AGI, and might be needed for other engines
in the future.
In order to do that, a new utility function was added:
`String::forEachLine` which gets a function as input, and its arg(s) (if it has any),
and calls the function on each line, and returns a new string which is all
concatenation of all the lines results (with '\n' added between them).
Continuing the work at https://github.com/scummvm/scummvm/pull/2236,
which moved `convertBiDiString(..page)` to unicode-bidi.h,
now moving to there also the `(..lang)` flavour.
Thus, translation.h has only the SVM-GUI related function, and the
two util functions page+code are in unicode-bidi.
