libretrans: API keys support and libsoup port

[mufeedali]

Lots of pending stuff but strictly speaking, it works.

TODO:

• [x] Fix code duplication.
• [ ] Add views in window for API key errors.
• [ ] Bring back error popover or implement an alternative.
• [x] Fix any other bugs.

[mufeedali]

@rafaelmardojai tell me if you see any glaring issues.