app: reset GimpFilterTool widget when resetting settings
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.
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