Signals from backend should be emitted only from backend.

Currently we emit Signaler's signals from the classes that gives functionality to the backend, we should code that classes using exceptions and maybe defers and let the backend classes to emit the corresponding signal. That way we don't need to pass out a Signaler instance, we know exactly which signals are emitted looking at the backend code and it's easier to document.

(from redmine: created on 2014-04-03, relates #4675 (closed))