make tgext.admin plugin work. this plugin requires toscawidgets2 to be activated

pboard/pboard/config/app_cfg.py

 base_config = AppConfig()
 base_config.renderers = []
 base_config.use_toscawidgets = False
-base_config.use_toscawidgets2 = False
+base_config.use_toscawidgets2 = True
 
 base_config.package = pboard