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.