gi: Only enumerate properties which GJS defines
We're currently enumerating properties which GJS doesn't represent/define like Callbacks, Signals, and Types.
Simple reproducer:
imports.gi.versions.Gtk = '3.0'; imports.gi.Gtk['CellCallback'];
This throws an error like...
typein:3:34 Error: API of type CALLBACK not implemented, cannot define Gtk.CellCallback
@typein:3:34
@<stdin>:1:42