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.

- now plugin is able to instanciate using the good child class
- the discover cache now contains plugin's type

Updated interface with nex dyncode

- Implemeting Client in plugins/webui/client.py
- Calling client in run.py

- webui now calls a hook to indicate a session end
- loader.py now set a lodel.dyncode module exposing dynmically generated code