KCupsRequest::waitTillFinished() is dangerous

KCupsRequest::waitTillFinished() works by spawning a nested event loop and using that to block the function until the operation completes.

Nested event loops are troublesome in general, and particularly in combination with QML. It causes crashes like https://crash-reports.kde.org/organizations/kde/issues/38387

We need to go through the usages of waitTillFinished and make the code properly async without nested event loops