before 4.4.0:

* 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.

eh, if i get to it:

* not sure how to make these coexist with the desktop menu:
  - "arrange items"/"auto arrange" menu option
* rubber banding?  probably not.
* make whether to show removable volume icons a pref (maybe hidden pref)
* make whether to show special file icons a pref (maybe hidden pref)