Added OnPreRegistration and OnInit extension events

[Kencho]

During the development of a project, we had to create almost a dozen stacked RT extensions. We would have found the ability to subscribe to RT events quite powerful in order to implement certain features that, in the end, we had to implement with workarounds.

This changeset implements the invocation of two almost-identical events during the extensions initialization, in case the extensions want to use them. They are completely optional, meaning no existing extension is required to implement such methods.