awful.key: Add more fields to the description

There is currently no centralized way to manage active keybindings so
the description data case be used to fill part of that role until an
official API is added.

lib/awful/key.lua

@@ -99,6 +99,8 @@ function key.new(mod, _key, press, release, data)
     data = data and gtable.clone(data) or {}
     data.mod = mod
     data.key = _key
+    data.press = press
+    data.release = release
     table.insert(key.hotkeys, data)
     data.execute = function(_) key.execute(mod, _key) end