Resolve "Printer panel doesn't return to empty-state after deleting last printer"

Show empty-state after removal of the last printer

The merge request checks whether the currently removed printer was last. It also checks whether there are other printer being removed by CUPS right now (their amount is still part of self->num_dests).

Do this check also in actualize_printers_list_cb() to catch corner cases from timing point of view. This will handle them with some delay if they show up. But this is very unlikely.

When undone, just check whether the number of printers is higher than 0 and show the printers-list page.

I don't use actualize_printers_list() by default as it introduces a delay to the update. "Directly" switching pages is faster.

Closes #2023 (closed)