builder: Add GtkBuilderScope

builder: Add GtkBuilderScope

GtkBuilderScope is an interface that provides the scope that a builder
instance operates in.
It creates closures and resolves types. Language bindings are meant to
use this interface to customize the behavior of builder files, in
particular when instantiating templates.

A default implementation for C is provided via GtkBuilderCScope (to keep
with the awkward naming that glib uses for closures). It is derivable on
purpose so that languages or extensions that extend C can use it.

The reftest code in fact does derive GtkBuilderCScope for its own scope
implementation that implements looking up symbols in modules.

gtk-widget-factory was updated to use the new GtkBuilderCScope to add
its custom callback symbols.
So it does it different from gtk-demo, which uses the normal way of
exporting symbols for dlsym() and thereby makes the 2 demos test the 2
ways GtkBuilder uses for looking up symbols.