Viewer event forwarding

[dlyr]

Viewer to plugin key and mouse event were requested in #677 and solved in #848
We may have dual behavior for event propagation, either with emitted signals or with callback registration.
Are both behavior needed ? if not which one should we keep ?

Currently, key event need to be registered with callback, and mouse event use qt signals.

[MathiasPaulin]

I do not understand why to restrict the question to viewer/plugins communication. I think that all events not managed by viewer should be transmitted to any registrated listener (application, plugin, object, …). Perhaps il is just a matter of wording but I think the precision is important.

[dlyr]

duplicate of #792 ?