Update yelp3 to 3.18.1

Highlights:
* Updated translations
* Fix accessibility regression for caret navigation, #754912 (Carlos Garcia
  Campos)
* Use XDG_CURRENT_DESKTOP, when available, to set platform conditionals (Shaun
  McCance)
* Fix crash after page title change bugfix (Shaun McCance)
* Fix page title changes when going backward and forward, #753448 (Carlos
  Garcia Campos)
* Improve subtitle to only be shown when it differs from the page title,
  #753444 (Michael Catanzaro)
* Fix startup activation, #753084 (Cosimo Cecchi)
* Use the correct background color, #749947 (Lars Uebernickel)
* Port to WebKit2 API, #686376 (Marcos Chavarr�a Teijeiro, Carlos Garcia
  Campos, David King)
* Use a header bar as a toolbar in Unity, #749903 (Iain Lane)
* Support for platform conditional tokens from os-release (Shaun McCance)

(prlw1)