Use-after-free of a full name editor when closing a contact editor

The EContactEditor added a signal handler to "editor_closed" to
close also a full name editor, but it did not remove the signal
handler on full name editor's destroy, thus the callback was called
on an already freed widget, which could cause a crash.

This had been reported at:
https://bugzilla.redhat.com/show_bug.cgi?id=1128745