Consider switching from gtk2hs to haskell-gi

haskell-gi seems to be better maintained than gtk2hs is. I think we should consider switching to haskell-gi.

There are some questions:

* Is cross-platform support of haskell-gi good enough? In particular how well is it supported on Windows?
* Is the extra runtime dependency (on GIRepository) a problem? Hopefully no.
* Are the bigger build dependencies an issue? Probably no.

The transition to haskell-gi means we're going to port ThreadScope from GTK 2 to GTK 3, which is not quite trivial.

Start with the migration questions in this issue: assess haskell-gi's Windows support, its GIRepository runtime dependency, and its larger build dependencies. Then evaluate the required ThreadScope transition from GTK 2 to GTK 3; the work is complete only when those risks are resolved and a concrete migration plan is established.

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