docview: DND enabled
docview: DND enabled

- dropping one or more files on the Contents treeview will open those
  files, in addition to what is already open
- dropping a file on the richview component will close all existing
  files and only open that single file.

wkitty42/fpGUI,wkitty42/fpGUI,wkitty42/fpGUI,wkitty42/fpGUI,wkitty42/fpGUI,wkitty42/fpGUI