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
Use AWTDropTargetEventDispatcher in Swing UIs
Support file lists better

These changes introduce some double dispatching that allows the code to
convert lists of files into a list of separate drop events. Right now it is
a little hacky in that it decodes file extensions and creates different drop
events as needed. This is not as clean as envisioned. We should think of
better approaches.
Implement lut and txt file drops
Break out code from private classes
Fix multiple minor bugs
Major change in drag and drop support

Drag and drop code now pass around Objects rather than exclusively
DragAndDropData. Each handler determines if the Object is a supportable set
of data. The FileList handler uses this capability to dispatch multiple File
drop events to various handlers. This should allow 3rd parties to add DnD
handlers that extend ImageJ2's drag and drop support seamlessly.
Remove obsolete comment
File list handler now supports List<File> input
Remove dead code
Ensure type safety in FileList handler
Remove obsolete ij-io dependency
Remove obsolete ij-commands dependency
Support more text file types in DND

This support more closely models IJ1.
Add some documentation
Merge branch 'master' into drag-and-drop
DefaultDragAndDropService: use latest SciJava API