Every GObject class we're ever going to make has the same handful of
methods. This adds *just enough* noise to be annoying. This commit
extracts the common ones so far into a central mixin. Since Zig is
removing `usingnamespace` and has no other mixin mechanism, we must
forward the decls, but this is still cleaner imo than what we did
before.
I suspect longer term we can probably abstract more of the `zig-gobject`
boilerplate into a higher level abstraction but I'm not confident doing
that yet across the 4 classes we have so far.
