More frame drag and drop fixes - SL #3819

- Errors introduced in recent commit 0aca8e59,
  jarabe.frame.clipboardobject.Format class doesn't have get_uri()
  method.

- jarabe.frame.clipboardobject.ClipboardObject: if format is
  'text/uri-list', Format.get_data() will contain only one URL.

Signed-off-by: Manuel Quiñones <manuq@laptop.org>