Update to 1.12.3

changes:
1.12.3
 - Use "c" instead of "v" for JSON contents of tagged element.
 - To/FromJSON instance: Use "t" instead of "tag", "v" instead of "val".
 - Use {"tag": "Str", "val": "foo"} format for JSON.
gtk3 -> gtk2; building with gtk3 is not considered stable and
turns out to be buggy.