AWTWindowEventDispatcher: tweak javadoc
Add imagej.ui.dnd package and event hierarchy

The imagej.ui.dnd package defines a new DragAndDropService which
provides a UI-agnostic way of handling drag and drop operations.

Similar to keyboard and mouse events, the imagej.ui.dnd.event package
provides a UI-agnostic set of events for receiving notification of
drag-and-drop events, which the DragAndDropService can utilize.

The imagej.ui.dnd package also defines a new DragAndDropHandler plugin
type to make drag-and-drop behavior extensible and customizable.
Support ImageJ DragAndDrop API in AWT common

There is now an AWTDropTargetEventDispatcher which translates AWT
DropTargetEvents into ImageJ DragAndDropEvents.

The Swing UI does not yet use it, though, since we need to chop up
the old AWTDropListener into UI-agnostic DragAndDropHandlers first.
Add drag-and-drop handler for image files
WIP: Use AWTDropTargetEventDispatcher in Swing UIs

This commit will be promoted from WIP to finished, and also remove the
old AWTDropListener, once all functionality from AWTDropListener has
been fully split out into individual drag-and-drop handler plugins.