awful.client.setslave: Fix for not-visible clients

Previously, setslave() put a client only at the end of the list of visible
clients. Obviously this means that it didn't do anything helpful if the target
client was not visible.

Fix this by iterating over all clients on the target screen instead of just the
visible ones.

Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/awful/client.lua.in

@@ -412,7 +412,7 @@ end
 --- Set the client as slave: put it at the end of other windows.
 -- @param c The window to set as slave.
 function setslave(c)
-    local cls = visible(c.screen)
+    local cls = capi.client.get(screen)
     for k, v in pairs(cls) do
         c:swap(v)
     end