scribusUnstable: Fix build with Poppler 0.84

see: https://github.com/NixOS/nixpkgs/pull/77186#issuecomment-583800213

it seems poppler updates (almost?) always break scribusUnstable

additionally:
formatted with nixpkgs-fmt;
added kiwi to maintainers - the more eyes the better;
add additional licenses that are listed on scribus COPYING;