Override the get_type_from_name() vfunc in BuilderScope, just like
the get_type_from_name() vfunc in Builder is overridden in gtkmm3.
Set a GtkBuilderScope with an overridden vfunc in Builder's default
constructor. Builder::get_widget_derived() can then (like in gtkmm3)
instantiate gtkmm-derived types without gtkmm__ prefixes in the .ui file.

Fixes #61