[PATCH wayland] data-device: implement drags with no data source for self-dnd

[Bill Spitzak]

Thank you for realizing that drag w/o data is useful!

This is certainly increasing my confidence that the Wayland developers 
know what they are doing, rather than copying decades of mistakes by X11 
and Windows.

Ander Conselvan de Oliveira wrote:
> Properly handle a drag with no data source, i.e., don't crash and send
> events only to the client that initiated the drag. This way a client can
> do self drag and drop without offering anything to other clients.