names: Split app initialization

Tests:

- When domain name is changed in config module. The updated names shows up in
the names module.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

plinth/modules/names/__init__.py

@@ -56,6 +56,9 @@ class NamesApp(app_module.App):
                                        **manifest.backup)
         self.add(backup_restore)
 
+    @staticmethod
+    def post_init():
+        """Perform post initialization operations."""
         domain_added.connect(on_domain_added)
         domain_removed.connect(on_domain_removed)