[PATCH wayland 1/2] add a dnd actions mechanism

[Gregory Merchan]

The XDND spec also has ASK and PRIVATE. http://www.newplanetsoftware.com/xdnd/

ASK allows a user to defer a decision until the last moment; to select
from a menu when the combination for moving, copying, or linking is
forgotten; and to be prompted before in irreversible action.

PRIVATE could allow symlinking instead of linking (or vice-versa), and
creation of a launcher instead of either of those. In XDND, the
semantics of the deleting files are covered under PRIVATE:
http://www.newplanetsoftware.com/xdnd/dragging_files.html . Whether
the intention is to store or incinerate, files are being MOVEd to the
trash. I don't know how much of this has to be at the Wayland level.

Other things which may or may not be covered by moving, copying, or linking:
* printing a file by dragging it to a printer icon. (Basically a copy?)
* sending a file to a colleague by dragging it to his or her entry in
a contact list. (Also a copy?)
* processing a file by dropping it on a script file. (Again, could be a copy.)
* opening a file with an application by dropping it on a launcher.
(Like the last one, really.)
* creating a copy from a template. (Copying but flipping an
is-a-template bit of metadata.)