client: Add wl_proxy_unref helper

Rather than open-coded decrement-and-maybe-free, introduce a
wl_proxy_unref helper to do this for us. This will come in useful for
future patches, where we may also have to free a zombie object.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Derek Foreman <derekf@osg.samsung.com>
Cc: Jonas Ådahl <jadahl@gmail.com>