Right now we have a somewhat byzantine process for saving files:
- we trigger a child process that saves the screenshot in /tmp as soon
  as the pixbuf is available
- we call into NautilusFileOperations to copy that tmp pixbuf into the
  user-specified file

The only reason why this would be needed nowadays is to support DnD from
the dialog to e.g. the desktop, or a folder. Remove support for that
(we're going towards a completely headless gnome-screenshot anyway) and
refactor the rest of the code to use simple async GIO methods instead.