dev-haskell/diagrams-gtk: Depend on gtk2

Specifically depend on a :2 version of dev-haskell/gtk.
dev-util/threadscope-0.2.3: Depend on gtk2

Specifically depend on a :2 version of dev-haskell/gtk, exactly as
threadscope-0.2.2-r2 does.
Merge pull request #320 from atlaua/pu1

GTK2 dependencies