wibox: Turn into "real" objects.

Before this commit, it was necessary to call 'rawset' to be
able to add new fields to the wibox. This is no longer required.

This solution was choosen because wibox is itself a base class of
menus and wibars. Those classes can now add new properties without
hacks.

lib/wibox/init.lua

@@ -26,6 +26,11 @@ wibox.widget = require("wibox.widget")
 wibox.drawable = require("wibox.drawable")
 wibox.hierarchy = require("wibox.hierarchy")
 
+local force_forward = {
+    shape_bounding = true,
+    shape_clip = true,
+}
+
 --- Set the widget that the wibox displays
 function wibox:set_widget(widget)
     self._drawable:set_widget(widget)
@@ -340,10 +345,18 @@ local function new(args)
     -- Make sure the wibox is drawn at least once
     ret.draw()
 
-    -- Redirect all non-existing indexes to the "real" drawin
+    -- If a value is not found, look in the drawin
     setmetatable(ret, {
         __index = w,
-        __newindex = w
+        __newindex = function(self, k,v)
+            if wibox["set_"..k] then
+                wibox["set_"..k](v)
+            elseif w[k] ~= nil or force_forward[k] then
+                w[k] = v
+            else
+                rawset(self, k, v)
+            end
+        end
     })
 
     return ret