awful.widget: Use private `args` for the update_function.

Previously, it would use the "real" args passed to the constructor.

It was a bad decision since:

 * It doesn't allow the tag/tasklist to add properties internally
 * It forces the widget to be created with a constructor rather than
   the alternate declarative syntax
 * It doesn't allow a tag/tasklist to be part of a widget_template

Technically this is a behavior change, but I doubt anybody will notice
given it is a dark and little documented corner of the API. Chances are
nobody have been using this API for years.

lib/awful/widget/taglist.lua

@@ -400,7 +400,9 @@ local function taglist_update(s, w, buttons, filter, data, style, update_functio
 
     local function label(c) return taglist.taglist_label(c, style) end
 
-    update_function(w, buttons, label, data, tags, args)
+    update_function(w, buttons, label, data, tags, {
+        widget_template = args.widget_template,
+    })
 end
 
 --- Create a new taglist widget. The last two arguments (update_function

lib/awful/widget/tasklist.lua

@@ -417,7 +417,9 @@ local function tasklist_update(s, w, buttons, filter, data, style, update_functi
 
     local function label(c, tb) return tasklist_label(c, style, tb) end
 
-    update_function(w, buttons, label, data, clients, args)
+    update_function(w, buttons, label, data, clients, {
+        widget_template = args.widget_template,
+    })
 end
 
 --- Create a new tasklist widget.