Avoid silent failure when adding one function twice as a listener for the same event

[Piotrek Koszuliński]

Currently events implementation accepts one listener function attached only once to one event on one object. When it's attached twice or more times (even with different context) it's ignored (or overwrite previous one).

To avoid issues like ​http://dev.ckeditor.com/ticket/9800#comment:11 events system should notify when listener is added second time. Perhaps we'll also find more issues like #9800, which will need to be fixed.

Harder way - we can remove this limitation from events system, by adding context as a next key. However:

• we would also need to change signature of event#removeListener (it'd need to accept context)
• there still will be a limitation about which developer should be notified by logged errors.

[Piotrek Koszuliński]

Duplicate: #9656.