printing: Free printer finder in callback

If a printer is not found during call of find_printer() then we need to let the freeing of printer finder happen in async callbacks of "printer-list-done" or "printer-added" signals as not all printers have been processed yet. I believe that all relevant scenarios are covered here including those when there are no backends or printers.

There are some failures in CI pipeline but they seems far away from the changed code.

Closes #4439 (closed)