Stop rendering all event objects when related models change

[colinsullivan]

Currently, all event objects re-render themselves when any field changes on a model. This should be different, and is likely the cause of slowness we are currently experiencing.

For each event type, its widget should only re-render when the segment's name changes (for example), because that is a piece of info that is being represented on the widget.

Backbone events have this facility I believe.