ecore_evas_convert: Implement hacky solution for Ctrl+C

ecore_evas_convert: Implement hacky solution for Ctrl+C

Problem: evas_object_image_save() can't be interrupted.
But, with ETC1/ETC2 encoding, it might be terribly long and
the user has no way to cancel encoding (except SIGKILL).

The proper solution would be to tell evas that the save
operation has been cancelled. And run that in a thread,
in a non-blocking way. I guess Efl.File.async could be
used for that, which means maybe also introduce
Efl.File.async_cancel? This will have to wait for
EFL 1.15 I'm afraid.

Note: edje_cc still can't be cancelled during ETC encoding.