TargetableEvent should extend RecordableEvent

Right now TargetableEvent does not extend RecordableEvent. This means that if an event implements TargetableEvent only the event is pretty much useless in XWiki standard. Moreover the documentation on https://extensions.xwiki.org/xwiki/bin/view/Extension/Notifications%20API/#HTargetableEvent gives the impression that it's enough to implement TargetableEvent only.
So it would be cleaner and easier for everyone if TargetableEvent would directly extend RecordableEvent.