Resolve "Possible refcounting bug around GtkListbox signal handlers"

We want to remove this warning in cases where the subsequent code isn't actually trying to access the underlying GObject and therefore isn't going to crash. On resolve() we don't want the warning, as that only applies to prototype objects. On trace() we don't want it either, as that may legitimately be called during the GC mark phase on a JS wrapper with a disposed GObject.

In the typecheck function, we only use the GObject for a debug assertion. All the other checks use the GType. We relax the assertion and only do the debug sanity check if the GObject is still alive. Instead of logging the warning here, we log the warning in gjs_g_object_from_object() which happens later when the GObject is actually used.

We keep the warnings in property getters and setters, since those would access the underlying GObject, and they are now only called for names that really do resolve to GObject properties and fields.

Closes: #24 (closed)