Add a gimp_filter_tool_reset_widget() function, which resets the
tool widget associated with the filter's controller -- i.e., it
resets those properties of the widget that aren't controlled by the
op's properties to some "default" state.  For most controller types
this is a NOP; for transform-grid controllers, we move the pivot
back to the center of the drawable, w.r.t. the current transform.

Call gimp_filter_tool_reset_widget() after resetting or reloading
the tool's config.