Support translating thematic packs

[tchx84]

e.g., tool that reads .entities (JSON) files and extracts "dialogue" fields, makes a .pot with that. Then, Gameeky server looks for $PROJECT_PATH/po/$LOCALE.po and applies gettext while reading dialogues fields before sending them to the player.

*NOTE that Gameeky clients are thin clients, they don't really know about the game logic etc, it's the server that does everything and streams the state of the game to the clients.