Mark order as complete doesn't work in Firefox if the label is not printed

[Ferdev]

In Firefox, when you purchase a label and check "Mark this order as complete and notify the customer", if you don't click on the "Print" button and instead just close the modal, the order is not marked as completed.

I think the problem is that the order is not marked as completed when the label is purchased, but when it's printed. It should mark the order as complete right after the purchase has been done.

This problem only happens in those browsers, like Firefox, that don't have a native way of displaying the print dialog and relies in a popup window to display the label's pdf.

[waclawjacek]

If this behavior is changed, it will affect the solution implemented in #2300.