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.

从这个 issue 中的迁移问题开始：评估 haskell-gi 对 Windows 的支持、其对 GIRepository 的运行时依赖，以及其更大的构建依赖。然后评估 ThreadScope 从 GTK 2 迁移到 GTK 3 的必要过渡；只有在这些风险得到解决并制定出具体的迁移计划后，这项工作才算完成。

由索引模型根据 Issue 内容生成。