GObject: add pseudo-classes for fundamental types

Add GObject.Int, GObject.Char, etc. that look like the JS fundamental
types but hold the correct $gtype property. This way, one can use
"GObject.Int" instead of "GObject.TYPE_INT" for GType arguments,
which is more consistent with GObject and GBoxed classes.
Also, attach a $gtype to String, Number and Boolean.

https://bugzilla.gnome.org/show_bug.cgi?id=722544