awful.menu: resize menu when item count changes

When the number of items of a menu change by menu:add or menu:delete
and the menu is shown, its wibox has to resize.

lib/awful/menu.lua.in

@@ -422,6 +422,9 @@ function menu:add(args, index)
         table.insert(self.items, item)
         self.layout:add(item._background)
     end
+    if self.wibox then
+        set_size(self)
+    end
     return item
 end
 
@@ -451,6 +454,9 @@ function menu:delete(num)
         end
         table.remove(self.child, num)
     end
+    if self.wibox then
+        set_size(self)
+    end
 end
 
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