Remove context on callbacks

[t-persson]

Description

We always call the callbacks with the event object and the context in which this event was sent.
This is a legacy feature that was used when prototyping the eiffellib and should be removed as it does not serve any really good purpose other than clutter in the callbacks.

Note that we need to start by deprecating it and then remove it.

Motivation

I have found no need for this feature at all and I don't see any real need for it anywhere else.
If there is a need for something like this, then we should instead add a utility module to eiffellib.
For example a function for fetching a link value from an event (get_link("context")).

Exemplification

Less unnecessary clutter in callbacks.

Benefits

Less unnecessary clutter in callbacks.

Possible Drawbacks

Removing this outright will break a lot of tools that rely on it.