protocol: define the concept of wl_surface role

Define what a role is, and what restrictions there are.

A change to existing behaviour is that a role cannot be changed at all
once set. However, this is unlikely to cause problems, as there is no
reason to re-use wl_surfaces in clients.

v2: give more concrete examples of roles, define losing a role, Jasper
rewrote the paragraph on how a role is set.

v3: make role permanent, there is no such thing as "losing a role".
Re-issuing the same role again must be allowed for wl_pointer.set_cursor
et al. to work.

v4: clarify the semantics of destroying a role object.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>