Xdnd callback (API Addition): Rationale below.

Xdnd callback (API Addition): Rationale below.

Essentially the problem is this: For drag and drop Ecore currently handles the
position update calls.  But usually the application wants to display some user
feedback - a window to display the drag selection for instance.

Now the way e17 does it is grab the mouse cursor movements and track them
itself.  However ecore is already doing this, it's already walking window
trees, calculating real positions, _and_ sending them in an X client message.
So it seems rather silly to duplicate the code in the user app to get the same
info.

We could possibly have added a new event, but then we need to deal the fact
the position update may arrive _After_ the item has been dropped.  Hilarity
ensures[1].

This callback is meant for purely the selection owner of the drag to use, so
it is a callback set, not an add.

[1] Segfaults probably.  Nothing funnier.

git-svn-id: svn+ssh://svn.enlightenment.org/var/svn/e/trunk/ecore@52181 7cbeb6ba-43b4-40fd-8cce-4c39aea84d33