Make add-on translatable

[lukaszgo1]

OCR add-on should be integrated to the automated translations system. At the moment it cannot be done for the new add-ons hence the blocked label. For the time being new translatable string should have translators commends. It would also help if the add-on could be build with a sconscript and not, as it's being done now, manually, but this is a topic for a separate issue.
After it is integrated *.po files for existing locales should be removed from the repository.
Related to nvaccess#4
Related to nvaccess/nvda#2744

[CyrilleB79]

Update:
Translation of add-ons is not formally blocked anymore from @nvaccess's point of view. Even if many add-on integrations in translation system are still pending (look at PRs in mrconfig repo), Windows Magnifier has been integrated successfully recently.

As explained on the mrconfig GitHub page (https://github.com/nvaccess/mrconfig), if you want to have your add-on translated, you need to ensure your add-on has a stable branch and ask for its translation on the NVDA add-on mailing list.