layoutbox: Update when a tag's screen changes (#1505)

In theory it would be enough to only update the old and new screen of
the tag whose screen changed. However, we cannot easily get the old
screen, so just update all layoutboxes.

Fixes: https://github.com/awesomeWM/awesome/issues/1503
Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/awful/widget/layoutbox.lua

@@ -48,6 +48,13 @@ function layoutbox.new(screen)
         boxes = setmetatable({}, { __mode = "kv" })
         capi.tag.connect_signal("property::selected", update_from_tag)
         capi.tag.connect_signal("property::layout", update_from_tag)
+        capi.tag.connect_signal("property::screen", function()
+            for s, w in pairs(boxes) do
+                if s.valid then
+                    update(w, s)
+                end
+            end
+        end)
         layoutbox.boxes = boxes
     end