This ports the config errors dialog from `apprt/gtk` to `gtk-ng`.
The major change here is that we now use proper template bindings for
the content. To do this, a `ghostty.Config` is now wrapped in a GObject
`GhosttyConfig` to make it safe to pass around (ref count) and to
provide helpful properties like the diagnostics buffer we bind to.
As a minor change, I stripped the `Ghostty` prefix from our GObject
classes in Zig code. For templates its all still there as is the norm.
This retains the exact same version requirements and layout as the
existing one.
7d99042070