Import bugfixes in settings validator. Hooks must be imported in validatore definition to avoid circular import failure

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.

Check that an EmClass.EmField exsists in dyncode

- Replaced a self. by cls. in leobject
- Changed default value for runtest option

- now datasources are declared with two conf keys : read_only and identifier
- the old datasources plugin hook is now declared in lodel/plugin/core_hooks.py. This file is imported after lodel/plugin/hooks.py in the loader.py

+ a new validator named 'string', it tries to cast the conf value using str()

To start in standalone uwsgi set lodel2.webui.standalone=True in conf file and start loader.py

- fixes exceptions output
- add "none" validator that validates only None