The json file is imported from the (MIT-licensed) emoji.json[0] node
module, which generates it from the emoji list published by the
Unicode Consortium.

This commit also adds a little tool to convert the data into
a compact GVariant, and the result of that conversion, which is
added to libgtk as a resource. The following commits will make use
of it.

[0] https://github.com/amio/emoji.json