TBlEventTarget: all methods to add event handler and filter handler should answer the new handler

[tinchodias]

In #443 we made TBlEventTarget>>#addEventHandlerOn:do: answer the new handler (this way, it is handy for senders to keep a reference to the handler and, for example, be able to remove it afterwards).

But we missed to do the same on other similar selectors in the trait, such as:

• addEventHandlerIfNotPresent:
• addEventFilterOn:do:
• when:doOnce: (but this also needs to be renamed and deprecated)

[tinchodias]

It's done for addEventFilterOn:do:, but it was not necessary for addEventHandlerIfNotPresent: because it already receives an instance of the handler.
It's also done for when:doOnce: but it's deprecated in favor of addEventHandlerOn:doOnce:, that returns the new handler.