* not sure how to make these coexist with the desktop menu:
  - "arrange items"/"auto arrange" menu option
* rubber banding?  probably not.
* 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.
* use a DbusGProxy to determine if there is anything in the trash, so we can
  grey-out the "Empty Trash" menu item appropriately.
* make whether to show removable volume icons a pref
* make whether to show special file icons a pref
