Add `Version` attribute to `Event` element so XML fragments can state the schema version they should be validate with

[at055612]

When we receive XML fragments, e.g. from the event-logging java library we just get a stream of

<Event>
  ...
</Event>
<Event>
  ...
</Event>
<Event>
  ...
</Event>

so we have no <Events Version="4.1.0"> to tell us what schema the xml adheres to.

Adding an optional Version attribute to Event would allow us to see the version for each event and potentially make decisions on. It would be optional as it is not needed in the case where there is a wrapper Events element.