In GimpFilterTool, bind the controller's lifetime to the lifetime
of the config object, rather than to the lifetime of the
corresponding tool widget; make the controller's widget pointer a
weak pointer to the widget, and bail out of the controller "set()"
functions when the widget pointer is NULL.

This fixes an issue arising when the properties of a config object
that outlives the widget change after the widget had died,
triggering a call to the controller's set() function, resulting in
a segafult when trying to access the widget.