Properties macro should error if construct(-only) property has no setter

[sdroege]

#[property(get, construct_only, default = 20)]
pub comments_per_page: Cell<u32>,

Gives

(lemmings:240427): GLib-GObject-CRITICAL **: 21:27:44.640: validate_pspec_to_install: assertion 'pspec->flags & G_PARAM_WRITABLE' failed
thread 'main' panicked at /home/dell/Projects/lemmings/build/cargo-home/registry/src/index.crates.io-6f17d22bba15001f/glib-0.20.1/src/object.rs:2420:13:
property 'comments-per-page' of type 'LemCommentsModel' not found

at runtime. Should probably simply detect and disallow that in the macro.