Builder: Using derived widgets: Update for glibmm 2.62

…which as of Kjell’s simply epic recent commits, now supports us setting custom properties of gtkmm-derived widgets in .ui files for Builder. Also, slightly elaborate the bit about why you would want to!

I also be optimistic here and presume that such properties could be made editable in Glade by using a custom catalog and property classes, though my Glade-fu is weak, so I can only hope I'm inferring that right.