Add InfiniGAG API (https://github.com/k3min/infinigag)

* Add a setting to choose between using 9GAG or InfiniGAG API, so that
if one of the API is not working, user can choose the other API as a
fallback
* InfiniGAG currently does not support: commentsCount, isVideo, isNSFW
* Add backported JSON library from Qt 5 to parse response from InfiniGAG
Fix wrong gag urls

Use back "data-url" attribute to get the gag url since 9GAG has
corrected it and the href from <a> tag has become a relative url
Hide the number of comments text when there is no comments