wibox: Stop monkey-patching the `get_children_by_id` function. (#2513)

Before this commit, it was added by `wibox.widget.base` if
`:setup()` is used. However it doesn't work for the `awful.popup`
because of the extra indirection.

This commit stops the monkey-patching and make sure the function
always exists. This doesn't prevent it from not working and in
the long run this should still be moved into the hierarchy.

However for now it makes the situation a lot more consistent and is a
quick band-aid without too much controversy.

Mitigate #2181

lib/wibox/init.lua

@@ -192,6 +192,19 @@ function wibox:set_screen(s)
     self._drawable:_force_screen(s)
 end
 
+function wibox:get_children_by_id(name)
+    --TODO v5: Move the ID management to the hierarchy.
+    if rawget(self, "_by_id") then
+        return rawget(self, "_by_id")[name]
+    elseif 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
+
 for _, k in pairs{ "buttons", "struts", "geometry", "get_xproperty", "set_xproperty" } do
     wibox[k] = function(self, ...)
         return self.drawin[k](self.drawin, ...)

lib/wibox/widget/base.lua

@@ -543,7 +543,9 @@ function base.widget:setup(args)
         rawset(self, "_by_id", ids)
     end
 
+    if not rawget(self, "get_children_by_id") then
         rawset(self, "get_children_by_id", get_children_by_id)
+    end
 end
 
 --- Create a widget from a declarative description.