implement (missing) support for setting custom labels

QFileDialogOptions::FileDialogOptions enable setting custom text, eg.
for the OK button, as in:

KFileDialog dlg;
dlg.okButton()->setText(i18nc("@action:button", "Copy"));

can be now ported to:

QFileDialog dlg;
dialog.setLabelText(QFileDialog::DialogLabel::Accept,
i18nc("@action:button", "Copy"));

CHANGELOG: implement support for setting custom labels in file dialogs