ecore_wl2: introduce offer api

ecore_wl2: introduce offer api

This commits adds api to deal with wayland offers.
It also ports elm_cnp to use the new api.

The selection_get and dnd_drag_get calls are replaced by simply receive
data from the offer.

The Offer object is now also emitted in every Enter,Motion,Drop and
Leave event, so a potential user can prefetch data and display it.
To finish a dnd operation positiv, the user has to call the finish call
before the offer is destroyed