Don't mix GJS and GObject signal systems

Don't mix GJS and GObject signal systems

GJS implements a basic signal system that allows monkey-patching
JS objects with signal methods resembling the GObject ones. However
it's clearly not a good idea to replace the actual GObject methods,
so use the proper GObject facilities when inheriting from GObject.

https://bugzilla.gnome.org/show_bug.cgi?id=778660