Use weak keys for referring to screens

This commit changes some tables that are used for per-screen stuff to have weak
keys, so that the screens can be garbage-collected, if needed.

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

lib/awful/widget/layoutbox.lua

@@ -47,7 +47,7 @@ function layoutbox.new(screen)
 
     -- Do we already have the update callbacks registered?
     if boxes == nil then
-        boxes = setmetatable({}, { __mode = "v" })
+        boxes = setmetatable({}, { __mode = "kv" })
         tag.attached_connect_signal(nil, "property::selected", update_from_tag)
         tag.attached_connect_signal(nil, "property::layout", update_from_tag)
         layoutbox.boxes = boxes

lib/awful/widget/taglist.lua

@@ -175,7 +175,7 @@ function taglist.new(screen, filter, buttons, style, update_function, base_widge
         end
     end
     if instances == nil then
-        instances = {}
+        instances = setmetatable({}, { __mode = "k" })
         local function u(s)
             local i = instances[get_screen(s)]
             if i then

lib/awful/widget/tasklist.lua

@@ -190,7 +190,7 @@ function tasklist.new(screen, filter, buttons, style, update_function, base_widg
         data[c] = nil
     end
     if instances == nil then
-        instances = {}
+        instances = setmetatable({}, { __mode = "k" })
         local function us(s)
             local i = instances[get_screen(s)]
             if i then