Add wrappers for setting the WidgetClass.activate_signal field

Add wrappers for setting the WidgetClass.activate_signal field

Setting a field on a class structure is not always an easy task from
languages other than C. While bindings can provide access to the class
pointer, twiddling the fields in the class structure can be awkward.
Additionally, signal ids are not always readily available.

We can paper over the direct access to the class structure, as well as
the "signal name to id" mapping with a simple couple of setter
functions.