brightness: control brightness

On the X1, the brightness is controlled by the OS. On other machines, if
it is not, we will double the requested changes. We can live with that.

lib/brightness.lua

@@ -14,7 +14,7 @@ module("vbe/brightness")
 
 local nid = nil
 local function change(what)
-   -- We don't really change the brightness, just report the change...
+   os.execute("xbacklight -" .. what)
    local out = awful.util.pread("xbacklight -get")
    if not out then return end
 
@@ -29,9 +29,9 @@ local function change(what)
 end
 
 function increase()
-   change("+ 5")
+   change("inc 5")
 end
 
 function decrease()
-   change("- 5")
+   change("dec 5")
 end