titlebar: Expose some previously private properties.

I am not happy with this API in general, I keep it undocumented
for now. In a perfect world, the whole client frame would be
a hierarchy.

lib/awful/titlebar.lua

@@ -513,6 +513,9 @@ local function new(c, args)
     -- Handle declarative/recursive widget container
     ret.setup = base.widget.setup
 
+    c._private = c._private or {}
+    c._private.titlebars = bars
+
     return ret
 end
 

tests/examples/awful/template.lua

@@ -94,7 +94,7 @@ local function client_widget(c, col, label)
     local l = wibox.layout.align.vertical()
     l.fill_space = true
 
-    local tbs = c.titlebars or {}
+    local tbs = c._private and c._private.titlebars or {}
 
     local map = {
         top    = "set_first",