Event chain

Currently, View::on_event receives a single event, not knowing who forwarded this event.

Knowing the parent chain can be useful to identify oneself (for easy self-finding callbacks).

In addition, it can help dealing with mouse events: even step of the chain would include an identifier of the child in the view, as well as its offset.

When receiving a mouse event, a view can subtract all the offsets from the chain to get a local position.