Completion: various clean-up
Completion: simplify update of info window contents
Completion: merge two callbacks as hide_completion_cb()
Completion: better handle key events
Completion: remove text_view_editable_cb()

It is useless to block interactive completion when the GtkTextView is
not editable, because when the GtkTextView is not editable, the signals
that trigger interactive completion won't be emitted.
Completion: always hide completion when insert mark moves