g_autoptr() is a macro that was defined in GLib 2.44 that allows for
basic auto-cleanup of variables. One way to add this kind of support
would be through the use of e.g. `G_DECLARE_DERIVABLE_TYPE()` for our
declarations, but this would consitute an ABI break (due to the
`...Private *` field in the public structs). Instead, we can use
`G_DEFINE_AUTOPTR_CLEANUP_FUNC` to manually declare this.

This commit also bumps the minimally required GLib version to 2.44