DefaultLUTService: update display when LUT changes

Otherwise, the color bar changes but the image doesn't refresh!
Add some comments in event dispatching sections
Broadcast DnD events to Swing DisplayWindows

This change only covers the frames though. Usually the DnD events also
need to be broadcast from UI components residing within those windows.
Broadcast DnD events from Swing ImageDisplays

Specifically, we listen for DnD events on the JHotDrawImageCanvas, and
rebroadcast them to the UI-agnostic drag-and-drop framework using an
AWTDropTargetEventDispatcher.

It is now possible to drag stuff onto the main part of an image display
and have something happen. In particular, you can drag a LUT onto an
image and it will change the image to use that LUT. Groovy.