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
More documentation changes

Also clojure file support