Test passes

- Child classes where not cleared when Plugin.clear() was called. Now child classes implements a clear_cls() method to clear a Plugin child class
- When discovering plugins if during a is_plugin_dir() check we import two times in the same virtual module name, all non existing attributes values are taken from prvious imported file (the bug was on plugin_type when webui was imported just before dummy, then dummy was an interface)

- The validator is written and added to all Plugin sublcasses confspec
- Adds the capabilities to give args to validator (here we give the expected type)

Created a new class LodelSession to handle session datas
Created an automatic wrapper to acces to session handler plugin function throug SessionHandlerPlugin class methods
Bugfixes done for plugins. The plugin type is store in __plugin_type___ and not in __type__