Add API to get the list of connected clients

This patch chooses the wl_list_for_each-style of iterating over
the clients, instead of using an iterator function, because i think
it is easier to use.

Signed-off-by: Giulio Camuffo <giulio.camuffo@kdab.com>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>