Hi Louis,
the idea with the ${haskell:Description} is that you put the description
in only once, in the Sources: stanza, see
http://anonscm.debian.org/darcs/pkg-haskell/tools/template-debian/control
Also, usually, if you depend on some c dev libraries in Build-Depends,
you also need to depend on them in the libghc-*-dev package. It might
work here as we well because the dependency libghc-gtk-dev already
depends on libgtk2.0-dev, but for consistency I suggest you add it to
the depends of libghc-gtk-traymanager-dev as well.
Also remove the comments from the Build-Depends template.
Greetings,
Joachim