Log Message:
po4a: update to 0.56.

        ___   ____   __
__   __/ _ \ | ___| / /_
\ \ / / | | ||___ \| '_ \      The D-Day release, 75th anniversary.
 \ V /| |_| | ___) | (_) |
  \_/  \___(_)____/ \___/      (released 2019-06-06)

AsciiDoc:
 * Introduce option tablecells to process cells in tables. (GitHub's #166)
 * widen accepted characters in the term part of definition list
   (GitHub's #169)

po4a tool:
 * Don't use single quotes to protect command line arguments, as it
   fails on Windows (GitHub's #141)

Translations:
 * New translation: Arabic, thanks mohammadA.
 * Updated: Dutch, thanks Frans Spiesschaert.
 * Updated: French, thanks Jean-Baptiste Holcroft.
 * Updated: Italian, thanks Marco Ciampa.
 * Updated: Japanese, thanks Victory.
 * Updated: Norwegian Bokmål, thanks Petter Reinholdtsen and Allan Nordhøy.
 * Updated: Portuguese (Brazil), thanks Rafael Fontenelle and Silvério
   Santos.
 * Updated: Ukrainian, thanks Yuri Chornoivan (українська).