Test passes

- Written a new custom_list validator, it takes another validator name as argument (in order to validate items of list)
- Using it to validate list of plugins names

- 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__

Implements both Plugin child classes handling extensions and session handling.
Modified the settings plugin preload piece of code. It is now adapted to the new plugins organisation.
Each Plugin child classes provides a confspec indicating where the plugin list is stored in configuration.