Correctly handle _NET_WM_DESKTOP in client_message_event()

[HarveyHunt]

A window may wish to be moved to a different workspace. This should be handled in client_message_event.

In order to do this, we need to be able to move an arbitrary client to a workspace. This can be achieved by factoring out the client {cutting,pasting} logic from op_{cut,paste} into their own functions.

_NET_WM_DESKTOP also needs to be added to ewmh_net_atoms.