the problem here is that we might subscribe to an event before
evas_object_callbacks_init has happened. This sounds like something
which might not happen. However, with the interfaces project this
definitly will start to happen because someone will some day overwrite
the evas object and do something before the constructor, which will not
raise a error or something but will simply just not work.
With this commit we are not listening to the event callbacks via event
emission but rather via inheritance. With this there is no "earlier than
we listend" point, and the issue in the task is solved by itself.