GUI: Removing an event with dependents (parents)

[rakhimov]

The removal of an event with dependents (gates, formulas) is non-trivial
because it potentially makes logical and type changes to the dependents to keep the model valid.

For example, what should happen (automatically) if the single argument of a NOT gate is removed?

1. Make the user remove the event from the dependents and handle the logic/type changes explicitly.
2. Provide an automatic logic/type change of dependents (implicit).

There may be more dependents that may not have automatic valid fall-back upon event removal
(e.g., event tree instructions).

Also, an element removal can have model-wise effect that may be hard on undo-redo stacks
and may end up being irreversable.