awful.layout: move arrange locks

This fixes a deadlock when some arrange signal handler itself changes
a property like `border_width`:

    function border1(c)
      c.border_width = 50
    end
    client.connect_signal("property::border_width", border1)

    awful.screen.connect_for_each_screen(function(s)
      local border_for_s = function(s_)
        local clients = awful.client.visible(s_)
        for _, c in pairs(clients) do
          c.border_width = 2
        end
      end
      screen[s]:connect_signal("arrange", border_for_s)
    end)

It might be better to have some counter there instead and emit a warning
if this happens (while allowing for e.g. 3 recursions).

This would allow handlers to break out of this themselves (by not insisting to
change the property).

lib/awful/layout/init.lua

@@ -232,10 +232,11 @@ function layout.arrange(screen)
                 c:geometry(g)
             end
         end)
+        screen:emit_signal("arrange")
+
+        -- Release locks after signal to avoid deadly lock.
         arrange_lock = false
         delayed_arrange[screen] = nil
-
-        screen:emit_signal("arrange")
     end)
 end