Replace some old pure python import by LodelContext calls
Change the plugins imports methods to use LodelContext instead
Fixes dyncode exposure method

Created groups (they can be found under the "Modules" menu -_- )
Created new pages : on LeAPI EditorialModel Plugins etc

- 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