Migrate PyGTK code to GTK+3.
PyGTK hasn't seen a release in years. The migration to GTK+3 introspection gives us plenty of advantages:
- Very similar syntax.
- New releases of GTK+ GI and updated documentation.
- Migration to Python 3. There are bindings for both versions and it nicely decouples the upgrade to the graphical widgets from the language upgrade.
- GTK+ has an actual web view where we can render proper HTML/JS in order to deprecate xdot graphs.
It's not clear if we can use this to refactor some of the code and simplify the on-disk structure.