docs: Tidy up GObject construction discussion in the GObject tutorial
docs: Tidy up GObject construction discussion in the GObject tutorial

Remove some outdated references to an old example, and add a row in the
table of steps in object initialization for the GObjectClass.constructed
virtual method.

https://bugzilla.gnome.org/show_bug.cgi?id=754855

ieei/glib,MathieuDuponchelle/glib,MathieuDuponchelle/glib,Distrotech/glib,Distrotech/glib,endlessm/glib,ieei/glib,mzabaluev/glib,johne53/MB3Glib,mzabaluev/glib,johne53/MB3Glib,MathieuDuponchelle/glib,johne53/MB3Glib,cention-sany/glib,Distrotech/glib,cention-sany/glib,MathieuDuponchelle/glib,mzabaluev/glib,endlessm/glib,mzabaluev/glib,endlessm/glib,MathieuDuponchelle/glib,Distrotech/glib,johne53/MB3Glib,ieei/glib,endlessm/glib,johne53/MB3Glib,mzabaluev/glib,ieei/glib,Distrotech/glib,endlessm/glib,ieei/glib,cention-sany/glib,cention-sany/glib,cention-sany/glib,johne53/MB3Glib