Implement elm_object_signal_listen and unlisten

Implement elm_object_signal_listen and unlisten

elm_object_signal_listen add callback(s) for edje object(s) of the widget.
elm_object_signal_unlisten delete this callback.
They're general functions, and every widget should set hooks for these.

It will improve elm extension flexibility. We have already
elm_object_signal_emit to send signals for the widgets theme, adding callbacks
is an expected step.

It provides a way to support sound on widgets. For example, in the elm extension
we could emit "file_to_play", "play", and add a callback with
elm_object_signal_listen for "*", "play". The callback function could use
the signal received to request the file to be played by the backend
sound system.

Certainly we should look for a better way to provide support for sound
on edje, but for now, it does the work. And anyway, it's only a use case
for these new functions.

SVN revision: 49868