Introduced WaylandClient API to keep track of clients.

Introduced WaylandClient API to keep track of clients.

Makes it easier to keep track of which surfaces belong to a given
client, and the client associated with a given surface. WaylandClient is
an opaque type, representing an underlying wl_client *.

Change-Id: If21a2e02eb13a860e6ac641875bdcca67a53fdf5
Reviewed-by: Laszlo Agocs <laszlo.p.agocs@nokia.com>