titlebar/wibox: Support widget definition in set_widget.

So now it is mostly identical to `:setup()` beside some legacy
difference in how the get_children_by_id is implemented.

lib/awful/titlebar.lua

@@ -492,6 +492,18 @@ local function load_titlebars(c, hide_all, keep)
     return true
 end
 
+local function get_children_by_id(self, name)
+    --TODO v5: Move the ID management to the hierarchy.
+    if self._drawable._widget
+      and self._drawable._widget._private
+      and self._drawable._widget._private.by_id then
+          return self._drawable.widget._private.by_id[name]
+    end
+
+    return {}
+end
+
+
 --- Get a client's titlebar.
 -- @tparam client c The client for which a titlebar is wanted.
 -- @tparam[opt={}] table args A table with extra arguments for the titlebar.
@@ -556,6 +568,7 @@ local function new(c, args)
 
     -- Handle declarative/recursive widget container
     ret.setup = base.widget.setup
+    ret.get_children_by_id = get_children_by_id
 
     c._private = c._private or {}
     c._private.titlebars = bars

lib/wibox/drawable.lua

@@ -13,6 +13,7 @@ local capi = {
     screen = screen
 }
 local beautiful = require("beautiful")
+local base = require("wibox.widget.base")
 local cairo = require("lgi").cairo
 local color = require("gears.color")
 local object = require("gears.object")
@@ -226,7 +227,7 @@ end
 
 --- Set the widget that the drawable displays
 function drawable:set_widget(widget)
-    self._widget = widget
+    self._widget = base.make_widget_from_value(widget)
 
     -- Make sure the widget gets drawn
     self._need_relayout = true

lib/wibox/init.lua

@@ -39,7 +39,8 @@ local force_forward = {
 --@DOC_wibox_COMMON@
 
 function wibox:set_widget(widget)
-    self._drawable:set_widget(widget)
+    local w = base.make_widget_from_value(widget)
+    self._drawable:set_widget(w)
 end
 
 function wibox:get_widget()
@@ -202,6 +203,7 @@ end
 function wibox:get_children_by_id(name)
     --TODO v5: Move the ID management to the hierarchy.
     if rawget(self, "_by_id") then
+        --TODO v5: Remove this, it's `if` nearly dead code, keep the `elseif`
         return rawget(self, "_by_id")[name]
     elseif self._drawable.widget
       and self._drawable.widget._private