`zig-gobject` is a set of GObject bindings that allow us to write
GTK-facing code in Zig instead of getting hands dirty with C. It's been
tested and refined in real-life applications and several GTK
contributors agree that it is a marked improvement over using the C API
directly, such as allowing method call syntax and avoiding many manual
`@ptrCast`s.
This PR doesn't actually contain any changes to our preexisting GTK code
— the migration process is intended to begin in chunks, firstly in
self-contained components (e.g. the header bar, overlays, etc.), and
then full-scale migration can begin when we remove non-Adwaita GTK
builds for 1.2. (After all, why port code that you'll remove later
either way?)
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