Custom UI + on-canvas controls candidates in GIMP

gistfile1.txt

1. Lens Flare: just moving a single handle around (the one used
   by Handle Transform tool and Blend tool)

2. Circular Motion Blur: Spiral-like, center + angle

3. Kaleidoscope: generally looks like a candidate with a basic gradient line, where:

   - inner handle is the center of the transform,
   - outer handle is Scale + Mirror Rotation.

4. Waves: we probably don't have a control for that yet, but the idea is:

- a control similar to the gradient line, but looks like an XY coordinate system, where...
- zero in X and Y is the center of the Wave transform
- X line is for the Period value
- Y line is for the Amplitide value

Just an idea :)

And there are some unported filters, like _Lighting Effects_, that would totally do
with on-canvas controls.