#869 engine optional in handlers

[sdesrozis]

Fixes #869

Description:

First attempt to discuss

• use of inspect to catch if engine is in signature

This is done in _fire_events. It should be better if engine was added in signature during registering. It implies a different order for arguments passed to _fire_events (basically Exception).

TODO : docs

@vfdev-5 thoughts ?

Check list:

• New tests are added (if a new feature is added)
• New doc strings: description and/or example code are in RST format
• Documentation is updated (if required)

[vfdev-5]

Some commented code to remove and add docs.
Otherwise, I tested locally and works like a charm :)

[vfdev-5]

LGTM! Thanks Sylvain !