PluginManager: Register plugin handlers when enabling account

gajim/plugins/pluginmanager.py

@@ -647,6 +647,9 @@ def register_modules_for_account(self, con):
                     continue
                 modules.register_single_module(con, instance, name)
 
+                for handler in instance.handlers:
+                    con.connection.register_handler(handler)
+
     def _plugin_is_active_in_global_config(self, plugin):
         return app.config.get_per('plugins', plugin.short_name, 'active')