Effects > Sunshine: add "color variance" slider

GIMP's "Supernova" tool does something similar, and it's a neat way to
make the burst look a little more natural.
FormMain: reorganize Effects menu

The Experimental menu is being downgraded to an actual Experimental
menu.  Any items undergoing testing can be placed in that menu, and once
they are working well, they can be moved to a permanent home.  Effects
remaining in the Experimental menu at release time will be permanently
hidden for that release.  This approach should make it easier to work on
new Effects, without having to wait until their 100% perfect to add them
to the menu system.

Also, a new "Lights and shadows" Effects menu has been added (name still
under consideration, and I don't want to use plain "Lighting" as that's
already an option in the Adjustments menu).  There are now sufficient
lighting effects to warrant their own category, especially with more
coming down the pipeline.
FormMain: reorganize Effects menu (additional files)
Effects > Rainbow: develop into full-fledged dialog

One of my projects for 6.4 is revisiting as many single-click effects as
I can.  If an effect is not meaningful enough to warrant a dialog of its
own, it will likely be removed; otherwise, it needs to be expanded into
a full-blown feature to match everything else in the Effects menu.

While this will result in a smaller Effects selection, the overall
improvement in quality is worth it.  PD's way past the point where it
needs to impress people with its Effects count, anyway.  ;)