Store localisation string files as readable Unicode text

Localised string tables are stored in Resources/Strings JSON files, but every non-ASCII character is escaped as \uXXXX. A French table shows Caf\u00E9 and a Russian table shows "\u0414\u043E\u0431...", so the files cannot be read or edited outside Objo Studio.

These files exist so they can be handed to translators, and escaped JSON is unusable for that workflow. Studio should store the values as literal Unicode text (UTF-8) so a localisation file can be opened, read, edited, and returned without any conversion step.