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.
Pass a "GimpCreateControllerFunc" to all gimppropgui-*.[ch]
constructors which takes a callback (to update the config object when
the on-canvas GUI) and a controller type that determines the
callback's signature, and returns another callback (to update the
on-canvas GUI when the config object changes).
In GimpOperationTool, pass such a GimpCreateControllerFunc that
handles creating and adding on-canvas controller via the new
gimpfiltertool-widgets.[ch]. So far, a simple line like in the
blend tool is supported.
Add a custom GUI for gegl:spiral, and have its origin, radius and
angle controlled by such a line.
Ell