+ bugfixes in show_object_detailled

get_referenced method were implemented for SingleReference and MultipleReference class
They are able to return LeObject instances refereced by them

Hook names were wrong

forget to name the argument

Now when we try to update an object that do not exists a 404 is answered and not a 500 anymore

Deleted old methods that are unused (one of them was triggerring an error will doing nothing)

Now we are able to limit the number of spawned child processes

Now listening address, listening port and max_children count can be configured using the Settings module

Adding a cache for app module reducing seriously routing overhead

Now all "empty" values are deleted from the resulting array

There were manual import through importlib, we have to replace them with LodelContext calls.
Actually the plugins module is in a dirty state....

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

**kwargs was not given to super() constructor

Implements a new exception class for webui : HttpErrors that uses a new template : errors.html to display multiples errors

Before we were calling the logging.getLogger function without argument. The result was
that all contexts were sharing the same logger (and every message were logged X time with
X the number of loaded context).
Now getLogger is called with the context id as argument, each context has its own logger.

WARNING : not fully tested

Now the loader context is threated like the others (it loads it's libs from a symlink, before it was loading them from lodel package)

a bit dirty & quick

They were a bug when ordering stuff contains 2 values that are equals

- a "normal" import was left in lefactory_common
- the hook triggering datasources init in dyncode was not called
- dyncode was not exposed in the loader (making hook call useless : the hook has to be called once the dyncode has been "seen" (imported) by python)

Regression may have been introduced about MONOSITE mode

concerning unittest & field validators

Fixes problems with plugins

+ uses the changes in the preload method