Fix DataMultisessionImportDialog No okClicked or cancelClicked signal issue.

K3b was developed by Sebastian Trueg started in 1998!
and K3b::DataMultisessionImportDialog was migrated by Michał Małek to KF5 in 2014!

There are okClicked and cancelClicked signals for KDialog.
Unfornately KDialog has been deprecated and moved to kde4support.
KF5 Prefer QDialog, but there is no such signal, and clang analyzer was not taught
to find such migration issue :)

Please (code review) pay more attention to K3b::DataMultisessionImportDialog::slotOk
and K3b::DataDoc::importSession, I prefer to use the backends provided by libburnia
developing and maintaining by Thomas if others failed to importSession!

CCBUG: 379268