… gimp_prop_widget_set_factor() to libgimpwidgets.

Now that GimpSpinScale is in libgimpwidgets, it's time to move the
associated prop too, to make it a prop widget with such a widget easily
creatable by plug-ins.

While doing so, I update both these functions logic, binding properties
together with the g_object_bind_property*() APIs (as we do already in
some other recent prop functions) rather than connecting to signals
ourselves. It makes for much simpler code.