element:event

There are 3 uses of element.

1. One or more token attribute's value might either be affected by a blockchain event or have its value origin from that event.

2. As the user assemble a transaction, the user-agent anticipates and observes an event to evaluate if the transaction was successfully done.

3. For a multi-step process (think a betting game), the event indicate that a game can proceed on and related players need to act.

Considerations.

1. <event> will need its own tag, for notification purposes and it has to include localised strings. It might trigger the availability of actions too.
2. <event> might trigger multiple attributes to update, therefore it should be definable outside of attribute-type. But there are local events that are only relevant for one attribute, therefore it's nice to have <event> inside an attribute type.