layoutlist: Add more caching.

Technically this doesn't solve any memory leak, but AwesomeWM uses in
average less memory when changing the selected tab in quick succession.

This is because it has less "temporary" tables to track.

lib/awful/widget/layoutlist.lua

@@ -118,8 +118,12 @@ function module.source.default_layouts()
     return alayout.layouts
 end
 
+-- Keep the object for each layout. This avoid creating little new tables in
+-- each update, which is painful for the GC.
+local l_cache = setmetatable({}, {__mode = "k"})
+
 local function reload_cache(self)
-    self._private.cache = {}
+    self._private.cache = setmetatable({}, {__mode = "v"})
 
     local show_text = (not self._private.style.disable_name) and
         (not beautiful.layoutlist_disable_name)
@@ -134,18 +138,22 @@ local function reload_cache(self)
 
     for _, l in ipairs(ls or {}) do
         local icn_path, icon = beautiful["layout_" .. (l.name or "")]
-        if icn_path then
+        if icn_path and show_icon then
             icon = surface.load(icn_path)
         end
 
-        table.insert(self._private.cache, {
-            icon         = show_icon and icon   or nil,
-            name         = show_text and l.name or nil,
+        l_cache[l] = l_cache[l] or {
             layout       = l,
-            screen       = s,
             callback     = function() alayout.set(l) end,
             style        = self._private.style,
-        })
+        }
+
+        -- Update the entry.
+        l_cache[l].icon   = show_icon and icon   or nil
+        l_cache[l].name   = show_text and l.name or nil
+        l_cache[l].screen = s
+
+        table.insert(self._private.cache, l_cache[l])
     end
 end