widget-factory: Add a discontinuous GtkScale with marks

So designers can tweak the marks and developers can see how a
discontinuos scale is implemented.

https://bugzilla.gnome.org/show_bug.cgi?id=741117
gtkscale: Draw smaller marks

As a design request, use smaller marks.

https://bugzilla.gnome.org/show_bug.cgi?id=741117