Introduce AbstractDropHandler

This abstract class represents an object that receives drag and drop
events.
This roughly maps to some of DataDevice's receiving methods, but by
being abstract
also allows us to use the same code from xwayland in a manner
transparent to all
code paths.

Preivous xwayland support with "proxy surfaces" have been removed.

--
In future it will be extended to have the mouse move events, so that we
can
avoid spying on the seat both in dataDevice and kwin's internal filter