Proper marshalling of flag arguments

Translate from the native representation of Flag arguments in the gtk
side (a CUInt typically) to a typed list of flags on the haskell side.
Make "cabal check" happier

And add myself as author/maintainer
Do not hide "catch" in the Prelude

Modern versions of base do not export it from there, so we do not need
to "import Prelude hiding (catch)" anymore. This removes a warning.