Add translations to appimage

[gyuris]

It would be great to add fresh translations to appimage on building: Translators can test their work.

Concept:

• pull .ts files from Transifex with our script: update_transifex.sh.
• generate .qm files from .ts files with lrelease
• place .qm files to /usr/share/scribus/translations/

It's not clear what should we do with directly maintained languages. It depends from #6.

[luzpaz]

Reference: scribusproject/scribus#78