Even when calling setClearButtonEnabled instead of setClearButtonShown
Recently KLineEdit::setClearButtonShown got deprecated, but that meant
that silently &KLineEdit::clearButtonClicked would not be emitted.
This patch corrects it. It uses quite a bit of internal magic, but
the old one was already using some so i think it's ok, given it even
comes with an autotest that will break when Qt changes its internals