awful.menu: sanitize support for access keys

Commit 87070ca introduced support for access keys in menus. Chosen
character was "&", an XML entity that has potential to cause problems;
if a user tries to add multiple access keys or just put an ampersand
in the name of the item the menu will break. Breaking happened because
ampersands were not escaped - first one was substituted and the rest
were ignored, but not escaped which triggered a pango markup error.

Signed-off-by: Adrian C. (anrxc) <anrxc@sysphere.org>
Signed-off-by: Julien Danjou <julien@danjou.info>

lib/awful/menu.lua.in

@@ -208,7 +208,8 @@ local function add_item(data, num, item_info)
     -- Create the item label widget
     local label = widget({ type = "textbox" })
     local key = ''
-    label.text = string.gsub(item_info[1], "&(%w)", function (l)
+    label.text = string.gsub(util.escape(item_info[1]), "&(%w)",
+                             function (l)
                                  key = string.lower(l)
                                  return "<u>"..l.."</u>"
                              end, 1)