Improve handling of fallback drop protocols

Improve handling of fallback drop protocols

* lisp/x-dnd.el (x-dnd-use-offix-drop): New value `files', which
means to only drop files using the OffiX protocol.  Make it the
default.
(x-dnd-handle-old-kde): Handle other data types correctly.
(x-dnd-offix-old-kde-to-name): New variable.
(x-dnd-offix-id-to-name): Fix typo in doc string.
(x-dnd-handle-unsupported-drop): Implement new value.