* move DnD handling to one place.  right now there are about four copies of
  it and they're mostly the same (and some of them are buggy).  need to
  preserve semantics where volume icon gets mounted before a file is dropped
  on it.
* need a way to determine if the *source* file can be dropped onto another
  icon.  currently we only check to see if the destination icon can accept
  a drop.  this is needed to determine if we can trash a particular file,
  and also so we don't try to move things like removable volumes.
* default move if file is on same volume, copy otherwise
* "arrange items"/"auto arrange" menu option
* rubber banding
* free placement (no grid)
* 'paste' action
* better tooltips for volumes and special icons (partially done)
* use thunar's file properties dialog
* progress dialog for copying/moving files
